The Steam version seems to have huge problems with alt-tab:ing under Windows 7. Doing it twice in the same session left a black rectangle covering everthing but the bottom 10% or so on my main screen. Even the Windows Task Manager was covered, so there was no easy was to kill it. I ended up running "taskkill /IM mirrorimage.exe /F" in the start menu search box to get back to the desktop.
Topic on Talk:Mirror's Edge
I ran into the game crashing almost every second time you alt-tabbed. Sometimes I was able to alt-tab more than 2 times before it crashed. I think it has something to do with the length of time it is minimized. I was able to open the task manager and ust it just fine every time though.