On a fresh lenny with virtualbox-ose and the modules just installed (both from the archive):
$ virtualbox
WARNING: /dev/vboxdrv not writable for some reason. If you recently added the
current user to the vboxusers group then you have to logout and
re-login to take the change effect.
You will not be able to start VMs until this problem is fixed.
Hrm, but I did do that...
Re-check:
$ groups
users disk cdrom sudo plugdev vboxusers sbuild