In VirtualBox 2.2.0, which was released today, that is. The new OpenGL acceleration for Linux and Solaris guests allows compiz to run very nicely in a virtual machine. (Click the thumbnail for a Theora video of compiz running in an OpenSolaris guest in OS X.)
EDIT: I suppose I ought to add there’s some other cool stuff in 2.2.0 as well, particularly the ability to import/export appliances in OVF format.
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