To try to speed up my Windows VirtualBox in a MacBook Pro running Mac OS X I intend to create a hard disk partition, same as for Boot Camp, and create a partition for Windows there. Would that speed up I/O? Is that something VirtualBox can handle?
Unfortunately VirtualBox has no support for partitions, and inasfar as I know VirtualBox does not allow you to use an actual hard disk partition - it only supports VirtualBox's own VDI format so far.
I'm running 3.0.8r53138, the latest version now, and I've been hunting for awhile now for exactly what you have asked for, to no avail.
In theory, it should speed up I/O operations, though the performance gains will be I suspect marginal (given my experience with VMWare Fusion + Bootcamp). Also, you might lose the ability to do snapshots, which is something I'd rather be able to do, then have marginally better performance.
P.S VMWare Fusion 3 is out now - i'm running it, and it's smooth like butter. You might to check it out.