I use a second monitor at work and only my onboard screen at home. After fiddling around with settings for a while, I got it to work, but I also shut down the system when I go to and from work (I expect restarting X would also work).

The main downside of this setup (other than the rebooting thing) is that it makes it impractical to unplug my laptop and move around when I feel like it.

I'm also interested in seeing what happens under Hardy.