Here is my scenario. The PC's are adventuring through a town filled with piles of dead fish. Within some of these piles are medium sized crabs which will ambush the PC's if they come close to their pile of yummy fishes. The question is when the PC's walk past the pile I assume they would make a spot check against the crabs hide check. But as the crab is hidden and technically not currently threatened could it take 10? Or does it have concealment? Or just a circumstantial bonus to hide? I am confusing myself here...
Hide checks typically aren't made until observation's possible
A creature (like a Medium monstrous crab) with concealment or cover (like beneath a Large pile of dead fish) makes its Hide skill check when potential observers have line of sight of to the place where the creature's hidden. Prior to the presence of any potential observers, it doesn't matter whether the creature's hidden or not (since no one's around to observe), so the game doesn't care, either. For this reason, taking 10 (and, similarly, taking 20) on Hide skill checks is normally impossible: a creature's Hide skill check is opposed by potential observers' Spot skill checks, and a creature trying to hide is usually "threatened or distracted" by the presence of potential observers.
However, taking 10 on Hide skill checks when time allows is a reasonable house rule—and one that this DM uses on occasion—as it gives some advantage to setting up ambushes. For example, a poor roll by one goblin raider no longer betrays the goblin raiders' presence, and lucky rolls by all the goblin raiders won't leave the PCs at a tactical disadvantage despite some PCs having invested heavily in the Spot skill. Note that this DM does not have a house rule that allows taking 20 on Hide skill checks—taking 20 typically involves multiple attempts to use a skill until getting it right, and allowing a creature to determine exactly the point at which it is perfectly concealed to the best of its ability is just too much for this DM. Nonetheless, a this player wouldn't reject out of hand another DM making a house rule saying differently, especially were observers present to provide feedback on the hiding creature's performance.