Land Spawning Pool enters the battlefield tapped. {T}: Add {B} to your mana pool. {1}{B}: Spawning Pool becomes a 1/1 black Skeleton creature with "{B}: Regenerate this creature" until end of turn. It's still a land. (If it regenerates, the next time it would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)1
8/1/2008: A noncreature permanent that turns into a creature is subject to the "summoning sickness" rule: It can only attack, and its {T} abilities can only be activated, if its controller has continuously controlled that permanent since the beginning of his or her most recent turn.
10/1/2009: The layering rules have changed. Now if you animate a land and it's power and toughness are lowered by Mutilate or a similar effect, reactivating the land's ability won't reset the values. This is because you apply all effects that set power and tougness to specific values before you apply any that modify power and toughness, regardless of the order in which the effects resolve.