VirtualBox OSE in Linux Mint 6
It turns out that the new kernel used by this version of Mint does not offer the precompiled VirtualBox kernel module packages which were available before and the instructions for embeding the module into the kernel via module-assistant just don't work, as is also the case for the setup command suggested by VirtualBox when it warns about the missing module.
Actually, after you install virtualbox-ose and virtualbox-ose-source, the kernel module is compiled but not loaded; you have to do it manually before you can open any VM. The command is: sudo modprobe vboxdrv.
I tried also the non-free version of VirtualBox, to see if it loaded the kernel module automatically; it does, but besides complaining about a missing USB server feature, it somehow fucked up my Windows XP virtual machine to the point that it won't boot anymore, even when I eventually downgraded to the open source version of VirtualBox.
So, finally, I decided to ignore all "official" instructions and be just practical: I added an entry to Preferences > Sessions > Startup Programs for loading the kernel module when I start my Gnome session by running the command "sudo modprobe vboxdrv". That's all it takes, fuck VirtualBox suggestions and module-assistant.
PD: of course, for this to work you have to previously configure sudo to work without asking for a password. I leave you that as homework ;-)