Hide Skill

The Hide skill, all in one place.

I'd like to point out that just about none of the things I'll discuss below have any official status. These are just the things I came up with to clarify the effect of the surrounding area and actions taken on the use and effectiveness of the Hide skill. If some official source is involved, a reference will be included.

Effect	Circumstance mod.
Up to ½ speed	+0
Up to full speed	-5
Faster than full speed	-20
Less than 50% conc.	No check allowed
50% concealment	+0
75% concealment	+5
90% concealment	+10
100% concealment	No check required
After a ranged attack (min 10 ft.)	-20
After casting a spell (min 10 ft.)	-20
No obvious effect or hostile target	+5
No Verbal comp.	+5
No Somatic comp.	+5

Casting a spell has certain mitigating circumstances that lessen the penalty (cumulatively).
Low-light or Darkvision may negate concealment.

Max distance between hiding places: 1 ft. per rank plus 1 ft. per point of magical bonus.
A Hide check takes a move-equivalent action, which can be combined with a move, to perform. (MotW, p. 19)

Movement
The effects of movement on the Hide skill are clearly described in the PHB: up to ½ base speed no penalty, up to full speed you take a 5 and moving faster gives you a whopping 20 penalty to your Hide check (PHB, p. 69). This assumes that suitable hiding places, or cover and/or concealment to put it in game terms, are available. When trying to stay hidden, the distance between hiding places cant be greater than 1 foot per Hide rank (add one foot per point of bonus from magic, like a Cloak of Elvenkind). (MotW, p. 20, S&S, p. 36))
When staying more than 60 ft. away from someone you are following, you only have to make one Hide check per 10 minutes. When you get within 60 ft. you have to make one every round. (MotW, p. 19) I think it is safe to assume this also means that, when scouting, staying more than 60 ft. away from observers makes one check per 10 minutes sufficient, wherease getting closer would require one check per round.

Concealment
The effects of the availability of concealment (there is almost no distinction between cover and concealment, regarding their effect on hiding, so from this point on read cover and/or concealment every time I speak of concealment) are less clearly described. In fact, the PHB doesn't even mention them in the Hide skill write-up.
Masters of the Wild and Song & Silence both agree that you need at least 50% concealment to hide succesfully, unless you have got some special ability like a Shadow Dancers Hide in Plain Sight ability.
d20 Modern, which is very similar to D&D 3e, does mention concealment. It says: "Cover and concealment (see page 144) grant circumstance bonuses to Hide checks, as shown below. Note that you can't hide if you have less than one-half cover or concealment."
In the accompanying table three-quarters concealment gives a +5 and nine-tenths a +10 circumstance bonus to Hide checks. Note that this favors stationary ambushes, as these can be laid in areas with a good deal of concealment. This tends to favor the PCs' opponents, more than the PCs themselves.

Concealment based on (natural) lighting conditions
Those characters relying on darkness alone, to hide them from unfriendly eyes, are in for a nasty surprise the first time they meet opponents with low-light or darkvision. For these opponents, the circumstance modifier from natural darkness based concealment should be lessened (or increased in the case of penalties). Darkvision negates such bonuses completely (even requiring a check for complete natural darkness, with NO circumstance bonus). Low-light vision reduces the bonus by one step (or +5), except for 100% concealment, as there has to be at least some light for low-light vision to operate.
Concealment reduced to less than 50%, by either type of special vision, makes hiding impossible.
Magically created lighting conditions tend to have their effect described (often making low-light and darkvision irrelevant). Those that don't, should follow the same rules as set out above.

The effect of movement on concealment
Determining concealment levels for stationary characters is relatively simple. Doing the same for moving characters shouldn't be much harder. You just take some sort of 'average' level of concealment for the path the character covers during the time unit for which the Hide check is relevant. For the mathematically inclined assign the values 1 through 6 to the different levels of concealment (None to 100%) and then take a weighted summation of those found in the path covered to get an average value. You might also just take the most common level as a base line and adjust that level's modifier by +/- 2, depending on the other levels of concealment.
Stretches of terrain without cover, longer than one foot per Hide rank (add one foot per point of bonus from magic, like a Cloak of Elvenkind), cannot be passed unnoticed (again excepting Hide in plain Sight and similar abilities).

Attacking from hiding
Those attacking with melee weapons, tend to leave their hiding places, so spotting them is an automatic success (the normal combat rules regarding concealment and miss chance apply, when hiding in fog/darkness), though comes too late to avoid being suprised. If the distance between the final hiding place and the target of the attack is less than one foot per Hide rank (add one foot per point of bonus from magic, like a Cloak of Elvenkind) you can sneak up and attack before he or she sees you. (S&S, p. 37) Those attacking with ranged weapons or spells, however, have a small chance of retaining their hidden status, given that there is enough concealment to hide behind. If you have already succesfully hidden at least 10 ft. from your target, you can make one ranged attack, then immediately try to hide again as a move action. You suffer a -20 circumstance penalty on your Hide check to conceal yourself after the shot. (S&S, p. 37)
That seems to imply for spellcasting from hiding that, if you have already succesfully hidden at least 10 ft. from your target, you can cast a spell that takes only a single action to cast, then immediately try to hide again as a move action. You suffer a -20 circumstance penalty on your Hide check to conceal yourself after casting the spell. (S&S, p. 37)
I believe that the presence or absence of Verbal and Somatic components, as well as how noticeable a spell is should affect your chance of staying hidden. So:
If the spell requires Verbal and Somatic components and the spell targets an enemy/area or has a visible/audible/tactile effect, then the Hide check would be at -20.
If the spell requires Verbal and Somatic components and the spell doesn't target an enemy/area and has no visible/audible/tactile effect, then the Hide check would be at -15.
Not having a Verbal or Somatic component lowers the penalty by another 5 points (cumulatively).
If the spell requires no Verbal or Somatic components (ie a Stilled, Silenced spell) and the spell targets only the caster and his/her allies and has no visible/audible/tactile effect, the caster need not make a Hide check.