I had this trouble and ended up removing the fglrx driver and using the open source driver. However it was not just as easy as uninstalling the fglrx file.
Here is how to do it easily and correctly. It will clean pretty much all of the fglrx info out of your pc and reinstall the open source drivers for you
Open a terminal and type
sudo /usr/share/ati/fglrx-uninstall.sh
this may or may not be on your pc
sudo apt-get remove --purge fglrx
This as you will see, get rid of pretty much all fglrx material on your machine
sudo apt-get remove --purge xserver-xorg-video-ati xserver-xorg-video-radeon
This will if you have installed it prior to installing 11.04 remove older open source drivers
sudo apt-get install xserver-xorg-video-ati
Installs the open source files
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
as does this
sudo dpkg-reconfigure xserver-xorg
Sorts it all out on your xserver which is currently blocking you getting into Unity
If you want to be able to reinstall the fglrx drivers at a later point (I have done before) you will need to reinstall the thing which prompted you to install them last time which is done by:
sudo apt-get install fglrx-modaliases
Now you will need to reboot and you should be back with the option to log into Unity again
Personally on my desktop I have now stuck with the open source drivers as I am always having issues with fglrx.
Hope that works