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In VB2.2.2 Ubuntu 8.10 host, Vista guest, nvidia 9600 hw, I see 3 different opengl applications where if you do Alt-PrtScn to get a window screen shot, what is copied are pixels behind the window, not the pixels in the window. If you run gotomeeting, the same thing happens the pixels behind the window are sent, not the pixels in the window. Anybody else experience this?

Does it do this if you shut off the 3D effects in the VBox GUI?
Also how much video memory does the host have and how much are you giving the guest

I have 64MB of 128MB video memory allocated, and when 3D acceleration is turned off, the problem goes away (but of course the openGL support is pretty crummy at that point).

I do not have the problem that you are experiencing but I see more tearing in Ubuntu 8.10 then I do in 9.04.
What is the kernel version and X version of your Ubuntu 8.10. Also I have found the changing some settings in Compiz makes things work smoother.
Try turning on Gnome Compatibility and adjust the Opacity settings in Compiz and see if this makes a difference.

I have tried turning off Compiz, special effects, various Gnome features, tried KDE similarly. I get the same behavior in all cases. Just to be clear, this is not every window, only openGL canvas windows. It does not appear to be application specific, namely, the three different openGL apps I tried, all have the same issue. My conjecture is that PrtScrn and gotomeeting all access the window's bitmap at a low level that is not supported by the current implementation. A plausible hypothesis?