I have friends that are Apple Geniuses. They say that Jail-breaking your phone will not void your warranty, unless it directly leads to the problem. For example, if I jail-break my phone and download an app that turns off my microphone jack, take it to apple and say the jack is broken, my warranty is void, however if there are no programs on the phone that are effecting the jack and it still won’t work, they’ll fix it, (barring of course, it was something that was covered by the warranty.)
The only reason you should fear Jail-breaking is the fact that anyone who programs something for it, can take whatever they want off your phone without you knowing.
If you submit an App to Apple and they find something that runs in the background that takes your contacts, email addresses, and stored photos, they won’t publish the App, however, as far as I am aware, there is nothing stopping people on Cydia from doing that.
If you don’t use contacts, email, or photos, or you don’t care who has that information, then totally jail-break! it is so awesome. You can customize EVERYTHING on the iPhone, even the Text Message Ringtone and, I haven’t checked myself, but my friend says that you can even install an MSM program, just like a regular phone, however it removes the apple default text program and replaces it with the MSM one, according to him.