Okay - before you try to play the beta - you should download the latest drivers. That goes for Nvidia, AMD and ATI. When you goto run the beta it will give a message which drivers you need to download should it detect that they are not upto-date. If you do not update you won't get full compatability, so it goes without saying that problems will occur.
All these drivers are located at the manufacturers websites so all you need to do is download then install (making sure you've selected the one the BF3 beta asked you to get.)
Then play. I will say that I was in a rush to play last night and managed to get it to full screen without the latest drivers. But maybe that's just me.
Anyway - for anyone that's interested - on the rig I played...