If you reset to factory settings and set it up as a new phone ie. not use a backup then it doesn't void your warranty as Apple won't know. If you rock up to the Genius bar and aren't there to mock them you should obviously make sure you've wiped the phone and reset everything.
This jailbreak lark is pretty well documented and you may want to keep firmware versions backed up too if you do do it. Generally I advise people not to, because those that should probably wouldn't be asking... generally.
If you reset to factory settings and set it up as a new phone ie. not use a backup then it doesn't void your warranty as Apple won't know. If you rock up to the Genius bar and aren't there to mock them you should obviously make sure you've wiped the phone and reset everything.
This jailbreak lark is pretty well documented and you may want to keep firmware versions backed up too if you do do it. Generally I advise people not to, because those that should probably wouldn't be asking... generally.
Its a fair amount of faffing.