I installed the Cimy Extra Fields plugin to get a customized registration/Signup page. Turns out that there seemed to be a bug with WPMU. I contacted the author, who will WPMU-ize the plugin with some sponsorship.
I am interested in contributing, if there are a few others....
The more of us there are, the less it would cost each of us.
The estimated total cost for the update would be about 120 Eur.
10 Sponsors would split, and cost 12 Eur each. Not bad.
The issue I run into with the plugin, is that it seems to work until you you get "too many extra fields" added into the registration page. For me it was 7-8 extra fields. This functionality allows a number of really cool things with blogs. In our case, we register new users and associate a number of data fields, then those are accessed in our code to display in the blog... user associated data, uploaded images, dropdown list chosen images, theme information etc... Very handy.
If you are interested, reply, and we can get in touch...