Although an iOS 4.2.1 jailbreak has been around for a couple months, it came with a catch: if you ever had to reboot your device, you'd have to connect (i.e. tether) it to your computer in order to start using it again.
Needless to say, people have been itching to get their hands on an "untethered" jailbreak (one that could survive a reboot). The good news: the untethered iOS 4.2.1 jailbreak was just released by GreenPois0n. The bad news: it's Mac-only for the moment, and GreenPois0n's site is currently offline.
Because I'm in the Windows camp, I haven't been able to test the jailbreak myself. And I'm not sure I want to, if only because iOS 4.3 is just around the corner. I will admit I'm eager to get back to using MyWi, the no-monthly-fee hot spot app. But that's pretty much my only motivation for jailbreaking at all.
If you don't want to wait for GreenPois0n to get back online, the jailbreak tool is available via Bittorrent. Keep in mind this is the Mac version; I'm not sure when a Windows version will surface.
As with any such tool, use it at your own risk. In most cases it's fairly easy to "un-jailbreak" a device, to restore it to its original, Apple-sanctioned state, but there's always a chance you could brick your baby.
Want to learn more about all this? Here's a great breakdown of the differences between tethered and untethered jailbreaks.
If you're a Mac user and you're able to snag the tool, hit the comments and let me know how it worked out. For everyone else, let me know your thoughts on the whole jailbreak scene. Worth it? Or not?