[ame=http://www.youtube.com/watch?v=WVZ5MDPVHWU]YouTube - Untethered 4.2.1 JB on ipt4g[/ame]
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.
@MuscleNerd's method can be generalized for use with all devices (including the Apple TV 2G) and is reusable for future jailbreaks. @comex's method is, he says, "more extensible" though.
No updates yet as to when this will be released, but we'll let you know as soon as we find out.