For those that desire the information, the 3.5 FAQ says:
"Do characters receive experience for killing a
summoned monster? What about undead created with
animate dead or an outsider called with planar ally?
As a general rule, any creature whose presence on the
battlefield is a direct result of another creature using one of its
special abilities (such as summoning) during the battle doesn’t
grant XP to characters defeating it. This is because the monster
is counted as part of the challenge provided by the summoning
monster. A pit fiend’s Challenge Rating (and thus the XP
reward granted for defeating him) already takes into account
the fact that he can summon allies; without that ability, he’d be
worth less XP.
However, there are plenty of situations where a DM should
make exceptions to this general rule. Any time that a creature
can bring an ally into play without reducing the resources it
otherwise brings to the fight (or well outside of combat) you
should strongly consider awarding XP for defeating that ally.
Let’s look at a few examples to see how this might work in
play.
Example 1: Over the course of many days, a powerful
necromancer stocks his lair with undead created via spells.
When the PCs fight the necromancer and these undead
minions, the necromancer has his full array of spells, so the act
of creating these undead hasn’t reduced the challenge he
provides. Thus, the Sage recommends awarding full XP for
defeating the undead.
Example 2: The same necromancer is on the run, knowing
the PCs aren’t far behind. He spends some of his precious daily
allotment of spells to animate a few zombies, only minutes
before the PCs bust down his door and attack. That’s a lot more
like summoning, since the creation of the undead represents a
direct drain on the necromancer’s immediately available
resources. Still, he doesn’t have to spend any rounds of combat
casting the spells, so it’s not quite the same. The Sage
recommends awarding one-half XP for defeating the undead.
Example 3: An evil cleric uses lesser planar ally to call a
succubus to serve him as a spy for 7 days, and sends her up
against the PCs (without being present himself). The Sage
recommends awarding full XP for defeating the succubus. If
the PCs then track down and defeat the evil cleric before he’s
able to prepare spells again, it’s tempting to reduce the XP
award for the cleric by a little bit (since he’s down one 4thlevel
spell), but it’s probably not worth the effort.
When in doubt, err on the side of awarding XP. The DM
shouldn’t be looking for opportunities to deny the PCs fairly
earned XP—if they bested the challenge, they should reap the
reward."
Anecdote: a PC that was away from the party pulled a bear out of his bag of tricks, but discovered he didn't need it so he sent it off to aid the rest of his party (without notifying them, either in-character or out-of-character).
The party meanwhile had recently defended themselves from an encounter with bears, so when they saw this new one they attacked it immediately and slew it. The DM awarded XP for it.