Exactly what it says. 'The version of the VirtualBox kernel modules
and the version of VirtualBox application are not matching.'
You may have upgraded one (or was upgraded during an apt-get upgrade)
and the other is still at the old version. Check their versions and
upgrade the other that isn't.
At the moment, e.g. using Synaptic, you will find that virtualbox-ose is
at version 1.6.6 in Lenny, and virtualbox-ose-modules-2.6.26-1-686 is at
1.6.2. That is the mismatch, which occurs now and then.
What you can do in such a case, is to compile the module yourself, in
the same way as it is explained for Etch here:
http://wiki.debian.org/VirtualBox?highlight=%28virtualbox%29
In short:
# module-assistant auto-install virtualbox-ose
# modprobe vboxdrv
# lsmod | grep vbox
vboxdrv 55344 0
And do an 'aptitude hold virtualbox-ose-modules-2.6.26-1-686' to
prevent the wrong module from replacing the new one during the next
upgrade.