OK, I managed to get the same problem by setting "RGB / TV Display" to "RGB" (the setting that works for me is "AUTO") I don't have a primary video BIOS setting anywhere. I'm running the 11/29/2006 - 0603 BIOS version.
With the setting on "RGB", the DVI port stopped working (until after the X server loaded), but the VGA port was still good. If you have a spare VGA cable, you might just try plugging that in. It would probably save a lot of hassle, because the keystrokes that worked for me may not work for you if you are running a different BIOS version.
Anyway... If you have "RGB / TV Display" set to "RGB", the following keystrokes should set it to "AUTO". There are three options, so if it doesn't work the first time, you might give it another try.
Code:
Immediately after the beep, hold down <DEL> for about 5 seconds.
<right arrow>
<ENTER>
<down arrow><down arrow><down arrow> (ie, press it three times)
<ENTER>
<up arrow>
<ENTER>
<F10>
<ENTER>