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.
Immediately after the beep, hold down <DEL> for about 5 seconds.
<down arrow><down arrow><down arrow> (ie, press it three times)