when I connect my netbook to my external screen with 1680x1050 resolution, it works quite fine in extended mode. The only problem is that the performance breaks down when you for example open expose with a couple of windows open. It's very slow and laggy. On the main screen, there is no such problem. Everything is working fluently.
You got this problem, too? If no, what Framebuffer kext are you using (which version? I'd be gradeful for a link!)
PS: I have 10.6.2 running, which is installed with NBI 0.83RC5.