Comment 6 for bug 769265

@Phearret, it is a VBox bug whereby the VBox code did not support any linux kernel above 2.6.37. So it was triggered by upgrading your kernel to a version higher than your version of Ubuntu ships with, which was higher than older versions of VBox support. The bug is fixed in VBox 4.0.4 according to their web site. But you can also work around the bug by using the kernel version that Ubuntu came with.

It's really not a bug in Ubuntu at all, although sometimes we do release Ubuntu updates to avoid such conflicts between proprietary drivers (usually graphics) and new kernels.