I had the same issue with 10.04 as well as 10.10 Beta.
This fixed the problem for both;
There was no existing xorg.conf file in /etc/X11, so I created one with
the following content;
Section
"ServerFlags"
Option "AIGLX"
"off"
EndSection
This is my hardware; http://paste.ubuntu.com/507411/
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[KM400]Haven't been able to execute normal gnome login since 10.04
installation; screen blanks then returns to login screen
https://bugs.launchpad.net/bugs/589520
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