Custom Fields on Facebook Registration

Any idea how custom fields (extended profile fields) can be added to the Facebook Registration process.

I spotted this registration: http://www.iphoneblogger.net/register/?fb_registration_page=1

And can see they have added the PMU functionality of registering for a blog... I guess it was added manually.

Say I have a field for Favourite Colour can I set up a blank field for the user to enter into?

Cheers

Elliot