Back in November, we first talked about redsn0w and outlined a bit of the downsides about trying it out. Those downsides were significant with the fact that the jailbreak uses techniques to set your iPhone free that didn’t leave a route back to normal and would void your warranty because Apple could tell you modded the phone.
Another important factoid in November when the tool surfaced was that it only supported the iPhone 3G and 3GS smartphones. The Dev-Team Blog is seeking redsn0w testers that want to try out a "backup plan" for them to the 4.2.1 untethered jailbreak. They are wanting these testers to help them find bugs before the new tool is offered in open release.
There are some caveats to the test program. They only want people using Macs and they want anyone who tests to not need the ultrasn0w unlock. If you are interested in using the new tool, you need to have 4.2b3 IPSW on your Mac.