It already seems like VBox on X knows about xinerama since it doesn't always fullscreen to both, or only to the first screen, so hopefully it won't be too much to ask to get vbox to see the host's entire display as one surface (I assume this is easier than properly handling the host's displays all independently...)
Thanks
p.s. I've noticed that XP boots and seems faster under VBox on linux, than it does on the bare hardware, thanks again ;D
edit: Just had another thought, if its possible, in seamless mode, make the windows task bar actually tell the X WM that that space is "used", just like any regular dock bar or window does? Just so kwin, and other WMs will notice that it probably shouldn't place any windows in that space?