Thank god ... I thought this was just me and my graphics card was going bad. I have had the same experience all weekend (9/10-9/11). The screen just freezes and I have to control+alt+delete to get out. I am able to log right back in (though it says I'm already logged on) and play for about 30 minutes if I'm in an iTrial, longer if doing less battle intensive activities. But it inevitably freezes again.
Posting this so you know it's not just you.
I am also running Windows 7, ATI Radeon HD 6800 Series, and updated all drivers, etc. just like you, but it had no impact.
Responders, thanks for the posts ... I'll try these suggestions, too!