I've got the same problem. No more than 30 seconds of game play at a time.

Okay, the problem appears to be resolved.
It could have been one of two things.

I had been playing on Experimental. When I updated to .49 the Stable servers were selectable in-game. Though obviously weren't working.
I also verified the game file cache through Steam and was apparently missing 1 file.
When I switched back to opt-out of Betas and acquired the missing file the game appears to be working.

Thanks for your help.

Hi J,
No other messages. Just 'session lost'.
Do I need to restart to ensure the update works? Someone else on feedback tracker said something about some missing files...?
Basically unable to play on any server for longer than a minute at the moment.
milo

http://steamcommunity.com/profiles/76561198069812720
Cheers