A tweet from @MuscleNerd last night revealed a workaround he's done for an untethered jailbreak of iOS 4.2.1 on the fourth-gen iPod touch. Using saved SHSH blobs from 4.2 beta 3, the method doesn't use expose any of the known holes that are used in the current set of exploits: it's a "backup plan" to the userland method that @comex is working on.