Edit: absinthe is updated to handle 5.1.1b now so this should no longer be an issue.
Today Apple released 5.1.1b for iPhone 4 GSM devices. If you manage to install this firmware you won’t be able to have an untethered jailbreak until a new version of the Absinthe 2.0 payload for this firmware can be produced. This will probably be sorted out in a few hours but you should all be aware in case you have an iphone 4 GSM (AT&T, for example) and have an issue. Apple did not patch the jailbreak and probably released this without being aware of the jailbreak. But a slightly different payload is required for it to work correctly with Absinthe v2.