Compare these two images (click to embiggen). The first is the live HDMI output. Hopefully you can see how it's slightly windowed, plus there's the red recording icon. The second is the actual HDMI playback, which fills the screen as you'd expect. The overall wonkiness is due to the wide-angle lens of my little point & shoot digicam.

Still, do you guys think there's a big chance a Magic Lantern hack for the T2i will emerge soon and enables HD-output during record?

It won't be soon. Tramm is focusing 100% on the new 5D2 2.0.4 firmware. After that, I believe that the 7D would be the next most likely. I have no idea if he even has a 550D/T2i right now, though he does have a 7D.

Yeah, I can confirm this. My Acer 23" does not play nice with my T2i via Mini-HDMI. When I hit record on the camera the monitor goes blank and the camera freezes up. Nothing is recorded.

First post and someone who is about to buy a 550d, this thread made me curious.

Is there any reason that you cannot capture the 1080i live view output?

edit: one more related question, is there any possible way of viewing the 550d hdmi output wirelessly? ie if its running along a cable or attached to an RC helicopter?

"Is there any reason that you cannot capture the 1080i live view output?"
if you speak about 5D or t2i, the answer is easy. There is no such output. The live output is not 1080i.
if you need to capture the video for remote monitoring, you can get the composite output and feed anything (cable, emitter) that fit your need.