Ubuntu 8.04 guest on VirtualBox 1.5.5 on Solaris: a problem with the screen resolution

I just installed an Ubuntu 8.04 guest on top of my VirtualBox 1.5.5 (with guest additions 1.5.6) on Solaris Express Developer Edition. As soon as I logged in, I wanted to change the screen resolution and I realized that the maximum resolution allowed was 800x600. It's not clear to me yet if the problem is related with the guest additions, but it was easily solved by adding:

Driver "vboxvideo"

to the device section. Doing so, Xorg now recognizes 1024x768 as a valid resolution, too. If you need higher resolutions you can add a Display subsection in the Screen section and manually specify the modes, as in:

Section "Screen"
Identifier "Default Screen"
Device "VirtualBox graphics card"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "2048x800" "1024x768" "800x600" "640x480"
EndSubSection
EndSection