I am running the new Mac Air 13" with OS X 10.8.2. I then downloaded the latest VirtualBox 4.2.6. Then, I installed Ubuntu 10.04 into the VM.
First off, after I installed the VirtualBox Guest Additions and restarted the VM, the Ubuntu desktop froze after the GDM login and I had to delete the VBoxGuestAdditions folders out of /opt and /usr/lib to get back to normal again.
Second, I go to Ubuntu's System > Preferences > Monitors and try to change the settings, and there's nothing higher than 800x600. I then tried the VirtualBox various View menu settings and tried again in Ubuntu's System > Preferences > Monitors and I'm still stuck.