The Dev-Team has just released the updated version of redsn0w for users who absolutely must jailbreak 4.2.1. According to developers, “very little works” (read: tweaks, hacks, etc), although the jailbreak itself works fine.
In the meantime, please make sure you have your 4.1 SHSH blobs for all your devices. These will be important even for firmware beyond 4.1 (using both comex’s method and our alternative, depending on how each of them turn out.)
Download it from one of the various mirrors here.