A semi-official build, straight from Samsung was leaked for the device and our friends at RootzWiki were able to get ahold of the system image where it can be flashed using Odin. Users are reporting that pretty much everything is working without hiccup, so we’re looking at a near-release build of Android 4.0 (with TouchWiz) for the Galaxy S II.
Installation is a fairly easy process but as Dima in our comments points out, this will mess with your bootloader. That being said, unless you have experience in these matters, it may be better off to wait until the device officially receives its update via Samsung/AT&T.