I just looked through Apple's forum a bit and found the following: Apple - Support - Discussions - MBPro won't detect external display on ...

Is that the issue? Seems like the OP had an older computer. Happily, this has never happened to me...

MBP and external display
 

Just to close the loop on this, as briefly mentioned earlier in this thread, I was having problems getting my new MBP to display on my Dell 24 inch LCD (2408WFP) via its second DVI port (the first DVI port is permanently plugged into my Win PC as I want to be able to toggle quickly from one to the other).

Could I get it to work, no way! I thought I was up against some of the glitches others had reported in the link posted above;

Then I found this support link.

How to use your PowerBook G4, MacBook Pro or MacBook with the display closed

Sounds straight forward. Then I tested the Dell LCD, all it's plugs and cables and found that the support method above WORKED, but only if I used the 1st DVI port on my Dell 2408WFP. The second DVI into the LCD is completely dead - which is why I could not get it to work before!

Dreading the thought of sending my beloved screen back to Dell right now (even though it has a 4 year guarantee) I have now reverted to using a DVI to HDMI adapter and now can plug the MBP into the Dell via that route (note the VGA input was another option but is really horrible by comparison to digital inputs). FCP on a 24 inch screen - lovely!