My external screen receives no signal from my laptop when I connect it on the laptop's mini Display Port. I use a mini Display Port to VGA adapter.
The mini Display Port is linked to a MCP79 (9400M) card, but the laptop also features a G96 (9600M GT) card. Traces were sent to email@example.com.
Another user with the same MCP79 card reported that he was able to get a signal, but the external screen would flicker; reclocking to a higher level doesn't help (see bug fdo#88272).