we deploy Centos and (K)Ubuntu sistems without problems. Are you try to upload a Ubuntu 10.xx image?
If you need deploy a ubuntu 11.xx image, you need make some changes:
*create or modify the menu.lst file
*install and configure the grub-legacy
See the next documentation (sorry only in spanish, but you can use Google traductor )
[url]http://www.ehu.es/es/web/instalaciones/fogehu/-/wiki/main/conocimiento[/url] (see Despliegue de imágenes con Ubuntu 11.04)