I use virtualbox (the non-free version) to run Adobe Photoshop Lightroom.
I had some new photos to import, therefor I tried to start Virtualbox.
I first tried version 1.6.0, however this version locked up my PC the hard way, without any clue what happened, therefor I quickly moved back to version 1.5.6 (accidentally the last non-Sun version) as this one worked beautifully before.
The kernel modules had to be regenerated, but a simple
/etc/init.d/vboxdrv setup fixed this.
However it turned out that virtualbox didn't start at all. There was a problem with one of the shared libraries it uses. Downgrading libxalan110-dev and libxalan110-dev by use of snapshot.debian.net fixed it for me.
See also this bug for more details.