Since I bought a mini-DisplayPort to DVI-D adapter, I decided to try it out on my 19″ Samsung SyncMaster LCD monitor.
Physically, the external monitor was to my left. However, by default, the MacBook thought it was to my right. Then, I discovered that, in System Preferences->Display, you could rearrange the logical position of the screens to match the actual physical configuration. After correcting the logical positions, everything went smoothly and behaved as expected. I could move the mouse pointer from the MacBook’s screen to the external monitor seamlessly. I could then move some windows over to the external monitor as well.
Spaces worked fine too, giving me additional workspaces on the external monitor as well. I could also move the Apple menu bar to the external monitor if I wanted to.
It would be interesting to have a 30″ monitor that displays 2560×1600 pixels. It requires a $99 mini-DisplayPort to dual-link DVI adapter though.