The focussing screen on the 7D (not the LCD, transmissive screen), is changeable. There is a regular screen below the LCD screen, which is accessible by removing 2 screws holding a plate, just behind top of the lens mount. The screen can be then removed using the usual tool that is supplied with Canon interchangeable screens (see focusingscreen.com site for images of this procedure). The shape & positioning of the locating lugs are different to the screens for say, the 50D or 40D, so existing screens from full frame cameras will have to be modified to suit (as they are for 40 & 50D's etc.). Katzeye is also currently developing screens for the 7D (see their website).
As a keen macro photographer, I really miss the 'split image with focussing collar' screen that I have on my 50D, so am looking forward to receiving the modified Nikon K3 screen for my 7D.
Kev