Wonder if adult billing companies would ever do this
 

So the surfer is at your join page hes all turned on, lots can happen from when he starts filling out the form, how about letting them have user name and passwords that stores their info, kinda like amazon and all sites that all you need is to login and click buy.

With that single username and password they can easily manage all sites they have access too etc.

i wouldnt make it a must for them to store their info but optional.

Then again this might not be something visa etc would like.