Eli Bendersky's website The HD space I allocated to my previous Ubuntu VM ran out, so instead of employing tricks to increase the HD, I decided to create a new VM and upgrade to 10.04 while I'm at it. All my data is quite easy to back-up and recover, and I'm still having the old VM around, just in case.
I downloaded a fresh 10.04 .iso from a torrent but it wouldn't boot in VirtualBox. After selecting "Install Ubuntu" in the welcome screen it would just hang with an empty black screen.
What eventually solved the problem was installing a new version of VirtualBox (3.2.8, while previously I ran 3.0.12). Re-installing VirbualBox was a painful process since it decided it must disconnect me from the network and it took a reboot to cause my network to work again.
Once installed, however, VirtualBox 3.2.8 installed Ubuntu 10.04 smoothly, with network, shared folders and guest add-ons all working very well.