I have been recommending Wubi to a lot of folks as a simple way to switch to using Ubuntu without having to worry about partitioning, etc. Wikipedia has a pretty good write-up on Wubi.
The problem, as I had expected, is that upgrading to a new release when available comes with a host of problems.
This discussion thread talk about the problem. It is fairly recent, so I don't think there is a solution for this yet. Does anybody have some different information?