To be precise, Nokia research engineers have ported most of the Ubuntu release (now including Universe) to ARM and it runs on the N800. See http://mojo.handhelds.org/

I have no idea whether they have tried to port the Ubuntu Mobile variant, but you could ask them.

A probably interesting exercise would be to compare the packages that make i.e. Maemo 4.1 and Ubuntu Mobile 8.04. See the commonalities, see the differences. I remember the Maemo SW engineer Eero Tamminen writing a very useful comparison between Debian, Ubuntu and Maemo but sadly I can't find it now.

btw, above there were links about Java missing probably the most important one: http://www.jalimo.org

PS: Recommending hardware selection and changes is just too easy when you are not the one doing the investment and being responsible of the choice. More here and here.