Please stop spreading misinformation. Jailbreaking does NOT "void your warranty," and I would know because I've had my iPhone 3G replaced several times while jailbroken, as well as my old iPod touch. They will likely refuse to accept your phone for replacement if your issue is software-related and they see a Cydia icon on your fully themed Springboard, but a quick restore takes care of that. In my case, I didn't even bother restoring - I simply disabled themes, hid a few icons using SBSettings/Poof, and that was it. My iPhone was/is still under warranty, because jailbreaking does NOT void your warranty.