I have the same exact problem.
I get random alt-tabs to my desktop from within the games I play. It happens so randomly that I cannot pinpoint what is causing the issue. It started happening a month or so ago. The only thing that has changed recently is installing two EVGA 680 GTX video cards in SLI. Before, I had two EVGA 580 GTX in SLI. There are many factors as well. During the past two months there have been a lot of updates with Windows, Nvidia, etc.
I have reformated multipled times to a clean system and updated my drivers and whatnot but the issue is still there. I even tried purchasing Windows 8 to see if it happens there but I reverted back to Windows 7 before I had a chance check it out. Very cool OS by the way. I reverted back for personal reasons.
Anyway, I started noticing it happen in WoW and just now Saints Row the Third played through Steam. It happens whether or not I am using my keyboard/mouse or if it is sitting there idle. I searched the net and many people are having this same issue all the way back from 2008 with different setups (computers etc). It happens so random and at different times. Some hours, days, weeks.
I wonder if Windows Event Viewer catches what is causing the alt-tab? Is there an application that tracks/records what is happening on your computer and tells you what processes were running at X time? That way next time it happens I can check it out there. I'm not sure if viewing my Task Manager when it happens would help. I wouldn't know which process was causing it without a lot of trail and error and if it was process that appears and then disappears.
Anyway, let me know if you find the cause of your issue. I'll report back if I progress in finding the cause of this.