Got the same problem here. Couldn't uninstall Apple Mobile Device Support (results in "fatal error during installation"). Version installed is v7.0.0.117.
Since I have nothing to roll back (won't risk destroying my other applications for this trash), I tried running “repair” on Apple Mobile Device Support (click on the “support information” link within the add-remove programs dialog to find it). That process stops with an error because the service won’t start. I clicked “ignore” and the process completed without further errors. Then I tried uninstalling Apple Mobile Device Support through add-remove programs. That worked.
It appears that all Itunes programs were successfully removed (can’t find the directories). Not sure if the library is still intact (two files wouldn’t copy when I backed it up before starting the update).
Just for good measure, I ran Tuneup Utilities registry cleaner to fix up some of what Itunes botched. It found about 50 issues with missing program files and other registry entries left over from the bad install, including an autostart entry for apsdaemon.exe. That was one of the first Apple programs that gave me (and others) errors when rebooting.