I think it would be more reasonable for user can login with OPENID, After he/she click the buy button then check if her/his address is filled. Otherwise he can use his/her OpenID to add product to wishlish, post on the forum...etc..
Some background: OpenId http://openid.net/developers is "a decentralized authentication protocol that makes it easy for people to sign up and access web accounts". It's a single identity provider and crucially it's supported by giants like Google, so a user arriving at a site who already has a Google ID can log in!
One famous site using OpenId already is StackOverflow.
Regrettably I've not used this before so can't vouch for it (nor guarantee I have the details right), but I think it might be something that NopCommerce should offer and perhaps should be a worklist item?
I've just written a membership provider for my employer's sites which logs people in against the NopCommerce database; if I have time and get the go-ahead I may look at adding OpenId in which case I shall report back!
Anybody implemented openId integration with nopc. I am looking at some idea to implement either openid or facebook login. please let me know if someone already implemented.