I've heard of people that say they use the built in speakers and/or microphone while they raid.
I've also heard of huntards that raid without a pet, DK's that equip spellpower gear, and seen many a talent spec where the player evidently randomly assigned their points.
DON'T BE AN IDIOT.
If you want to raid, please do yourself and your raidmates a favor and get a decent headet with a functioning mic.
I've seen blog posts arguing vent (voice communication) isn't needed/preferred for raiding. That person belongs over in the corner with the huntards/deathtards w/e.
Good communication is what makes a raider/raid solid or even exceptional. You won't be any good if you're typing instructions in raid chat. You need to coordinate your efforts and notify the raid of specific events, even if you're not the raid leader or a 'designated talker.'
You need to be able to notify healers if you get certain debuffs, and its very useful to be able to warn others of void zones or instant kill events.
The decent headset is necessary to hear raid warnings and proc events. It's even more important to anyone using audio warnings for cooldowns and the like (but that's a blog for another day). A stereo headset may not be required, but I find the experience more pleasant, and sometimes knowing where a mob or fire or what have you is, is very helpful.
So, at a minimum, pick up a decent headset with stereo sound and a microphone. You can get cheap on-ear headsets for less than $20 - so please do yourself a favor and buy one!
Logitech Precision Gaming Headset
Gamecom 377. Good luck finding the headset that 'fits' you best!