What are the limits on summoning. Can each character have something summoned at the same time? I've got 2 clerics, a sorcerer, and a wizard, so I'd like to do 2 animate deads, and 2 summon monster X. Will it work or will the entire party only be able to have 1 thing summoned at a time?

Summoning limit is six.
It doesn't matter who casts how many summons or if the summons is from a spell or an item (eg. wolf figurine from docks barbarian). A party can have a total of six summons only at its disposal.
If you already have 5 creatures summoned and cast a summoning spell which gets you 2-3 summons (Summon Monstar I) the spell casting fails and you get a message that your summoning limit has been reached.

Thanks Shrikant!