Laptop freezes when starting X -without- external monitor

When in the office I use an external monitor in dual screen mode (with Ubuntu 11.04)

To go out, I need to disconnect the monitor and run off the internal screen only.

When I do this and the try to boot, X starts, then there is an arrow pointer that moves for approx 1s, then the machine freezes - no keyboard, no mouse, just a black screen with the arrow cursor immobile either right in the middle or wherever you left it if you try to move it for the 1s prior to it freezing.

On other unix/linux platforms I'd be able to configure the system so X wouldn't start and then login to the console, reconfigure it (ati-config in my case) then start x manually.

I have absolutely no idea how to achieve a similar result in Ubuntu.