I'm running on Vista Ultimate 64 with a Geforce 8600 GTS.  Frame rate is great, but if I try to Alt-Tab out, the game crashes and I never get back.  If I try, all I get is a big gray screen to look at.  The game is still running, but the video is permanently screwed.

I have the same problem with Supreme Commander, so I think this one is GPG's fault.  I sure wish they'd fix it.

Well, given the amount of time that I spend trying to connect or waiting for others to connect to a custom game, alt-tabbing helps me keep my sanity.  Even if the networking were flawless, it can take a while to get enough people to join a custom game.

I should also mention that I haven't had this problem with any other game.

I could do that, but windowed mode is known to decrease graphics performance.  I've yet to see a game where that is not the case.  It's a pretty game, and I'd like to play with all the settings turned up, as I do now and it looks and runs great.

I suspect that this Alt-Tabbing problem is just some DirectX setting or callback that is not being handled correctly, and the problem seems to be present in both GPG games that I own.  That tells me it's something in their shared graphics engine and it would be nice to have it fixed.