I came across this excellent tutorial at the Mac|Life magazine website to allow you to install Linux onto a Mac and have it dual-boot. Rather that repeat the article here, I'll just post the appropriate link;
Here are some screen shots of my setup just to prove that is IS possible, and worked very nicely too !
1. The External Monitor
Forgive the lines, but that's what happens when you use old technology (i.e. an external CRT monitor)
2. The MacbookPro LCD Screen
Much clearer, just to prove that the LCD *does* work !
3. The (out of focus) Login Screen
Because I'm running dual screens, I generally only use the external monitor when I'm at my desk, so the login screen appears there only. If I disconnect it, then the login screen appears on the MacbookPro's LCD as you would expect.
4. And to prove that it all works !!
The one annoyance I haven't been able to resolve, simply because I haven'y had time, is having two different wallpapers.
I have had email from friends who have tried this method with their Macs (Mini's, Macpro's, etc) and tell me it all works flawlessly.
I'd be interested to hear if anyone else tries it.