[huwshimi]

Here are my thoughts on registration forms:

You should always have absolute minimum number of fields required to start using the service.

Don't have any optional fields (there are plenty of better places to ask for that information).

If there is information required to use certain parts of the service then ask for it when they want to use that part.

http://tumblr.com and http://posterous.com examples of this. I'm actually a fan of the single password input too... I implemented a similar form recently: http://hellohype.com/accounts/register, only with a username field instead of a url field.

I'm not a big fan of going to the extreme of Instapaper though. They don't require a password at all, so a malicious user can access and modify another user's account.