I know I made a big deal about the initial pre-alpha of the hack, but this new release is 10-times more important
But seriously, the latest update (found at the ml-devel Google Groups site) to the ML firmware for the 550d is now available. Be absolutely sure NOT to update your camera to official firmware 1.0.9, or this won't work... yet. There is no way to reverse an official firmware update.
Anyway, the big deal with this release is working in-camera menus, which means users are now able to change gain settings on-the-fly. This was the one missing feature that was keeping the T2i from being a fully effective run-and-gun machine, in my opinion, so this is really a huge step.
On top of that, here are some recent additions included in the latest release:
Cropmarks (4:3, Anamorphic, Fisheye)
Full-Manual, realtime gain control over both preamps (analogue and digital)
Zebras for over- and under-exposure
Automated rack focus (like the XL, XH, and XF pro lines)
Mixed input (internal mic, ext. mic, l ext, r int)
Headphone output (through USB)
Plus a host of other bugfixes.