I installed Ubuntu; after the installation I restarted the VM, and during the "restart" VirtualBox crashed. At that point, VirtualBox would start, but hang (no window would open, just a Dock icon); this was true both starting from a double-click on the icon, and starting VirtualBox from the terminal.
The various VirtualBox command-line items still appeared to work... just no GUI. I tried uninstalling/reinstalling, still no luck.
Solution:
1. Uninstall VirtualBox
2. Shut down & Restart
3. Restart VirtualBox
I suppose this probably has something to do with the way VirtualBox acts as a kernel extension, but really, no idea.