In the US it's always legal to do:
http://www.intomobile.com/2010/07/26/iphone-jailbreak-and-android-root-declared-legal-in-us-feds-exempt-smartphone-hacks-from-dmca/
As far as handset warranty, rooting voids it, but you can always restore to stock unrooted if something isn't working right and you need to take it in for warranty service. If it completely breaks, they'll likely just replace it, and wont be able to see if it was rooted or not.
If you do root, definitely think about custom ROMs.
I too have been thinking about rooting my Android. I find all the stock apps that run in the background annoying. Things like Facebook, Skype, Myspace, City iD, ThinkFree Office I don't need or have any use for them so I'd like to get rid of them. But, I've been afraid to do it because I'm not sure how hard it is and I don't want to brick my phone. Though, I do have a warranty and the phone could, "Have a very unfortunate accident" if I do brick it.
Anybody have an easy to follow walk-through on how to do this?