A friend of mine has a proggy to lock his mouse in one monitor, just wish I could recall the name of it at the moment. He might see this and pipe in. If not I'll ask him later and try to get back if you hadn't found a good solution by then.
I have been using two monitors for a couple of years and have found very little trouble with mouse straying even without any additional proggys. Most games in fullscreen on primary mon. will lock the mouse on their own, only problem spots being a few that use archaic edge scrolling. Even then most have alternative and better ways to scroll the map like middle mouse button dragging or something so I rarely ever use edge scrolling.
Edit/Addition: He said he was using ultramon, but when he switched to Win7 64bit it didn't work with it at the time. May have a version up now/soon though.