Got quite a few friends, as well as myself, having this issue.
It's a
fairly big one, as it can quite severely detriment the playability of the game for anyone running more than 1 monitor which will probably be a lot of serious gamers (most of the gaming communities I've run are filled with people using a second monitor for teamspeak etc) who have the potential to bring in more players.
Right now we have people, turning off their computers, unplugging their second monitors, restarting... just to play the game competitively. But that can't last.
I did a little looking around and potentially this issue can be caused by other programs, so if you're running Raptr, XFire etc or anything you don't need, it might be worth closing these to see whether they resolve the issue.
If this doesn't work, someone on the Steam forum wrote a small VB6 app to lock the mouse to the primary monitor. Download and use at your own risk, but it seems to work and contain no malware
http://s382701517.online.de/apps/PrimaryLock.zip