When the "vboxdrv" driver for the Linux kernel is loaded, after a patch by Red Hat's Dave Jones, it will mark the kernel as tainted crap. Even though this VirtualBox driver is open-source (it's under the GPL), the quality of the driver is quite poor and continues to cause issues for many users. In particular, kernel developers have become frustrated that this virtualization driver is causing random memory corruption. Specifically cited is "corrupt linked lists, corrupt page tables, and just plain 'weird' crashes."
The code comment for the patch mentions, "vbox is garbage." The VirtualBox kernel driver is needed for providing some features to guests on this Sun/Oracle virtualization platform. While the VirtualBox kernel driver is open-source, it doesn't live within the mainline kernel tree and is distributed separately with the VirtualBox software package.
Marking this driver as "crap" puts it in the same boat as the staging drivers and select other modules that are of less quality than the fully-supported in-tree modules. With this tainting, some bug reporting tools will not report these VirtualBox problems or handle any bug reports in a different manner.
It's not hard to find Linux users having a troublesome VirtualBox experience. I for one was briefly a VirtualBox user with an Ubuntu guest from a Mac OS X host last year on a production machine, but then switched to VMware and the experience has been tremendously better than the buggy Sun/Oracle software often causing lock-ups and other problems. In the forums are many more VirtualBox stories. In addition to being more stable, another benefit of VMware is their virtual graphics driver for 2D/3D guest acceleration is superior, which utilizes Mesa's Gallium3D.
The discussion about this VirtualBox tainting can be found from this kernel mailing list thread.