By Bad Seed
By @stroughtonsmith and @chpwn.
A public release is not possible at this time for a LOT of reasons. So please don't ask.
EDIT: Also, unfortunately, we can't explain how it's done — it's also not possible just using publicly available tools, so that wouldn't be useful anyway.
(Once there is a public jailbreak for the iPhone 4S, this should become somewhat easier to distribute. No ETAs there, though: I have no clue when that will be.)