As always with a new version of Autocad (R2011) , it is a good idea to update your video driver.
I am lucky enough to have a quadro FX1500, so I duly updated it.
Somewhere along the line, and I'm sorry I cannot remember the exact sequence of steps needed, this all came unstuck. I had text that disappeared after moving, slow everything, viewports that went all funny. Just beore I thought to blame Autocad, I decided to check 3dconfig. There it had a choice of 2 drivers- One Autocad and one that looked like an invdia one.
I changed to Autocad and all was fixed.
Hope this helps someone!