Alright, so this a bit nuanced, but I thought that I'd share it anyway. Maybe this snippet will help someone somewhere.
I'm using the Signup module to permit users to signup for events, but I'd like them to be able to enter more than the standard name and phone number. I'd like my users to be able to provide their address.
Beyond that, if the user has already provided their address on their user profile (generated using Content Profile and the Location CCK module), then I'd like the fields to prepopulate.
Furthermore (we're getting crazy here), if they enter a new address on the signup form, I'd like them to be able to save that address to their profile.
Actually, with the power of the Forms API, this becomes an easy task.