Can I "branch" VirtualBox snapshots (base multiple VM on the same disk image + separate diffs)?

I keep multiple VirtualBoxes for different projects. They are all running the same Debian base system. Keeping complete copies of all the disk images is getting expensive.

Is there a way that I can base multiple VM on the same (read-only) disk image, with (hopefully small) diffs layered on top? This is just like snapshots work, except that there is no concept of "current" and "previous", they are more like branches.

Or can I just use snapshots (of the "same" VM) for this? I would be wanting to run more than one of those snapshots concurrently.

As of VirtualBox 3.1, this is possible. See http://www.virtualbox.org/wiki/Changelog-3.1; the branched snapshot capability is listed near the bottom of the page, under "VirtualBox 3.1.0".