The problem disappeared after the following steps:
1. Uninstall any package that is related to VirtualBox OSE: virtualbox-ose, virtualbox-ose-guest-tools, x11-org-driver-virtualbox-ose
2. Reinstall the VirtualBox Guest Additions
3. Restart the guest machine (the resolution is automatically set to 1280x800)
4. Now, 1280x800 resolution is read by saX2