I've found using the non-OSE VirtualBox 2.0.2 amd64 version that:
* XP in a VM still has this problem, although if you move the mouse over the XP task bar, after a second the Ubuntu desktop returns (as soon as you move it away from the taskbar, the XP desktop background redraws itself).
* Vista in a VM works correctly in seamless mode with compiz on (so long as I don't have XP running in another VM). Vista of course is so slow as to be almost unusable compared to XP.
I've found using the non-OSE VirtualBox 2.0.2 amd64 version that:
* XP in a VM still has this problem, although if you move the mouse over the XP task bar, after a second the Ubuntu desktop returns (as soon as you move it away from the taskbar, the XP desktop background redraws itself).
* Vista in a VM works correctly in seamless mode with compiz on (so long as I don't have XP running in another VM). Vista of course is so slow as to be almost unusable compared to XP.