I just wanted to chime in here. I've had the external monitor issue with my 15" Macbook Pro, Core i7 2.66ghz, 4gb of ram that I purchased in late July. My computer was a refurbished model, that I think was produced sometime in spring. When connected to my 21" Dell at work, it's fine for about 10 to 20 minutes, and then both screens go black. Nothing revives the computer other than holding down the power button to hard reboot it. I've tried everything from closing the computer to unplugging the adapter and just waiting for it to come back. Twice I've been to the genius bar, and though I have saved every single crash report, and they agreed they could see something going on that wasn't right, when I took it in to be serviced (the genius bar guy said it would probably be a logic board replacement) they could not replicate it.
At this point, I'm starting to wonder if this might actually have more to do with a faulty display port adapter than the computer. With out the display port adapter, I've only crashed in twice; once zooming in using the track pad while in Photoshop CS4, and once while playing HalfLife 2 through steam. I will be taking my computer in again to the genius bar, and will continue to do it until they resolve this issue. This time I'm going to get them to use my mini-displayport adapter (it's display port to DVI by the way) to see if they can replicate it and maybe get a replacement. I don't know if anyone else has looked into replacing the adapter to see if that resolves the issue; that's what I'm going to try to do. I'll post back to let you guys know if that resolves anything.