I have VS2010 and CodeRush Express installed on my machine. Occasionally I am hitting some key combination (inadvertently, don't know what I am hitting) that is causing my up/down arrow keys to get "stuck", they stop moving the cursor up and down. Restarting the dev env fixes the problem.
Just had the same problem. I managed to put editor back in "normal" mode by Ctrl+Tab-ing to different tab and back.
Hope this helps someone.