My iPhone has been jailbroken for a couple days (iPhone 3Gs) 4.2.1 with Green Poison rc6 and literally, a few minutes ago I had to reboot for a cydia app and it got stuck at the Apple/Skull logo.
Mind you, this happened at 7:17 PM.
I just tried the basic force restart.
Yeah, obviously your phone is a bit upset or something. Unless you've done other recent upgrades or installations, I think Quickreply might've been the problem. Perhaps it's not compatible with 4.2.1 yet. Use biteSMS! In my opinion, it's better... and it works on 4.2.1!