I have a Linux Mint 10 Host with Virtual Box 3.2.8 OSE installed.

I have installed Ubuntu 11.04 Beta as VBox guest but the VBox Guest Additions that come with VBox 3.2.8 won't compile for the X11 on Ubuntu 11.04 so I was restricted to 1024x768 resolution.

My monitor supports a higher res. So to get a higher res in the Ubuntu 11.04 guest the answer is:

# install the vbox x11 module available 
# from the standard repositories in the guest
sudo apt-get install virtualbox-ose-guest-x11
# reboot

Update Ubuntu 11.10

# install the vbox x11 module available 
# from the standard repositories in the guest
sudo apt-get install virtualbox-guest-utils
# reboot

There was one weird thing once I did the above and rebooted the guest. The guest would only give me 1280x960 instead of my actual physical monitor resolution of 1280x1024. But it's better than what I had before.