Some time ago, a CentOS install on a particular new Dell workstation needed the "nomodeset" kernel parameter to get a graphical login screen to correctly display. This was with a Radeon Firepro 2260 graphics card.
After applying the CentOS 6.3 updates, Xorg wouldn't correctly detect the LCD panel's resolution anymore. A 1680x1050 panel would get a 1280x1024 resolution. In the days of digital DVI connections, DDC probing and what not, this was unusual and surprising.
A lot of searching and testing led to the solution: the nomodeset parameter broke Xorg probing. Rebooting without the nomodeset parameter worked (no graphical problems like I had earlier), and solved the Xorg resolution probing.