So the biggest reasons to jailbreak is if you want to unlock it (for international travel, for example), or you want access to Apps which Apple doesn't approve of (backgrounder, tethering on AT&T), or you want to modify the GUI to make your iPhone more unique (using Winterboard), or you just want to geek around with it and bring up a Unix prompt on your phone.
Winterboard is nice - being able to replace icons and change the background, adding 5 rows and columns and make your iPhone look uniquely yours is a nice benefit.
SBSettings is slick - you swipe across the top row and you can access features that are normally several menus down in the Settings icon.
Backgrounder is still relevant with the new 3.0 release. If you want to play Pandora and still use your phone to do other things, it's a useful addition.
I don't know if it really voids the warranty or not... For what it's worth though, it's easy to undo a jailbreak. Just upgrade or restore to any Apple firmware release and the jailbreak disappears.
I will say that every new major release from Apple seems to take more and more of the reasons to jailbreak away. Before the 2.0 release, Jailbreaking was the only way to get apps onto the phone. Then before 3.0, it was the only way to background stuff, the only way to MMS. Now with push notifications and integrated MMS, there's not that many good reasons to jailbreak.
If you decide to try it, google the iPhone dev blog and then click on one of the links for "redsn0w" which is the new jailbreaking app for the 3.0 release.