The Dev-Team's original blog post about the redsn0w/limera1n cocktail just keeps getting bigger and bigger. Update number 3 to the post includes links to version 0.9.6b3 of redsn0w for OS X
and Windows for non-ultrasn0w unlockers:
We‚Äôve made some updates to redsn0w to make it easier for jailbreak developers (and tinkerers) to get their programs ready for 4.2.1. As noted above, the public version of Cydia (and MobileSubstrate too!) is not 4.2.1-compatible. redsn0w will now let you install your own custom bundles independent of Cydia (the bundle can actually be Cydia if you‚Äôve compiled it on your own). These bundles can be up to 15MB in size, and should be in the form of a gzip-compressed tar file.
Note: The Cydia that‚Äôs included in 0.9.6b3 is the same one as in 0.9.6b2, and so it will *not* work on 4.2.1. Don‚Äôt try installing it on 4.2.1! Instead, use the SSH bundle, or compile Cydia on your own. If you‚Äôre familiar with the apt utilities, you can use ‚Äúapt-get‚Äù to install many programs from the command line. Be sure to do ‚Äúapt-get update‚Äù first to refresh your sources!
PLEASE CONSIDER THIS AN ADVANCED TOPIC!! It‚Äôs not meant for the masses because it involves rather nerdy things like command lines and tar files. But for those who know how to use this new redsn0w feature, have fun!