warranty is stil valid, all you need to do is to flash it back to the original firmware then make the claim. dont go and make the claim with your IPhone Jail broken. Make sure to restore it back to the original firmware then make the claim.

If anything wrong occurs during flashing etc etc, there's always the life saver option of putting the phone into DFU mode and reflashing it. It's almost brick free in that sense.

i jailbreaked my sisters ipod touch 2nd gen other day, and mid way through flashing, screen blank, buttons dont work, cant be turned on. i thought i bricked it. really worried until i remembered DFU!! saved the Ipod Touch. Redsnow is an excellent application, it will teach you the step by step process in real time. 4-5 mins, JB Iphone!! Fun begins!