I have a 20" external monitor hooked up to my MBP 15", which I am using as my Windows XP screen (full-screen parallels). It works really well, mousing between MacOS X (laptop screen) and Windows (external monitor) The only thing I am noticing (and I'm wondering if it's just my setup) is that every so often I see a "flash" on my external screen while I'm working in MacOS X, which catches my eye. When I look at it, nothing's changed. It's as if there's a noticable refresh going on in Parallels. Has anyone else seen this?
I have seen my friend's MacBook Pro 17" do this when PD is in fullscreen mode. He runs PD more than I do; I haven't seen it on mine, but I don't use it fullscreen very often. So you are not alone!