Yep; the last upgrade not only broke something, but gave me a bug that's been hanging around since at least February, where you get stuck at the gdm login screen in an endless loop...
This occurred after an apt-get upgrade and apt-get dist-upgrade from Lucid Beta-1 to Beta-2 on Friday...
When I boot to GDM, everything appears to be working. I can switch to a console, and login as normal. But if I click on a username in GDM to login, GDM freezes then restarts itself. It seems to be triggered by the appearance of the password field. If you choose to enter a username you can do that fine, but as soon as you move focus to enter a password, GDM restarts: X-server quietly crashes under the nVidia drivers!
This is definitely nvidia-96 specific as many users have reported. I am using the Nvidia-96 driver, on a Geforce4 MX440 AGP card - ok, it's elderly, I know, but not that elderly and no reason for Lucid not to support it.
So I used scalawag_'s suggestion to get to the Lucid Desktop.
At the gdm screen:
Ctrl+alt+F2 - starts a terminal session
login with username and password
sudo service gdm stop - stops gdm graphical front-end
In my case, it launches into a gnome-failsafe session-mode, because that's what is specified in;
As of yesterday, 18Apr, still awaiting bug fix in nVidia drivers.
UPDATE: Forcing an Update Manager check and upgrade just kicked me into a 250mb (!!!) update by way of a massive update to the Lucid Beta-2...