Today I installed the latest alpha of Ubuntu 10.04 Lucid Lynx Alpha inside of VirtualBox. After getting all of the updates I installed the VirtualBox Guest Additions. Upon boot my screen flashed lots of fun colors and then told me that my display configuration was fubared.
I booted up in low graphics mode and then searched for the answer, which I'm going to repeat here.
After installing the guest additions you need to modify a file.
gksudo gedit /usr/src/vboxvideo-3.1.4/vboxvideo_drm.c
Replace line 90 with the following:
#if defined(DRM_UNLOCKED) || LINUX_VERSION_CODE >= KERNEL_VERSION (2, 6, 33)
.unlocked_ioctl = drm_ioctl,
.ioctl = drm_ioctl,
Finally, run the following command:
sudo /etc/init.d/vboxadd setup
So anyways, this got me as far as logging in without the graphics dying. However, it looks like Compiz gets enabled and kills everything. I'll update if I find a solution to this.
Edit: Bug 6198 is fixed as of VirtualBox 3.1.6. To get it for Ubuntu add the VirtualBox PPA found at http://www.virtualbox.org/wiki/Linux_Downloads and run sudo aptitude install virtualbox-3.1