I installed Ubuntu 11.04 on 2 laptops today. One of a friend, and one of my own. Both have NVidia graphics cards.
This went the same on both computers. When you login the first time you get a message wich is basically telling you to install the NVidia drivers.
However, when I restarted after installing the new driver on my friend's laptop, Unity just worked.
When I installed the drivers on my laptop, and rebooted, it just booted straight back into the Ubuntu Classic desktop environment.
In the sessions list at the login screen, there are the following items: Ubuntu, Ubuntu Classic, Ubuntu Classic (no effects), User defined, Recovery console. I haven't touched this setting, as it seems to me that Ubuntu should be correct.
Appart from that when I login I still get the Ubuntu Classic environment (without the message telling me that Unity couldn't be started) When I run
compiz --replace manually, I get the Unity interface... but it instantly freezes.
So why doesn't it start the Unity interface? My graphics card is an Nvidia Quadro NVS 110M, wich has always worked fluently with Compiz Fusion.