There is a fledgling Edubuntu port to the XO, created using the same basic mechanism as Installing Debian as an upgrade and using some steps from http://eclecti.cc/olpc/ubuntu-mobile-on-an-olpc-xo. You can install this via:
# olpc-update -r edubuntu
Log in as user 'xo' with password 'xo' when the machine has rebooted. You can boot into your previous Sugar installation by holding down the circle gamepad key as you boot.