I turned on solution-wide analysis and a single aspx file was marked with an error. Loading it up in VS2008 causes the entire GUI to stop responding to mouse input.
I found the error manually in another editor and it was a simple missing double quote on a style tag (").
Removing this allowed me to open the file and continue editing the file. It's very odd that it caused R# to compeletly stop any mouse and keyboard input to VS2008!