Have a Dell OptiPlex 980 with (2) ATI Radeon HD 4550 video cards and 3 monitors connected to it (23" primary and 2 secondary).
At seemingly random times, the 2 secondary monitors just disconnect and shut off (not power, just signal). This seems to happen mostly at idle times, but it just happened again while in use. When it happens, I have to go back into settings and re-extend to them again and configure location, resolution, etc.
I've updated to the latest drivers via Dell's website, which were dated April 2010.