If I reach http://musicbrainz.org/user/nikki/contact I have no easy way to go back to nikki's profile.
How about the browser's back button?
If someone sends you an email via that form, the email contains a link which takes you directly to http://musicbrainz.org/user/username/contact
Ah. If the use case is to go back to the user page, perhaps a direct link to the profile page would make more sense in that context than a cancel button?
Why not just keep the profile navigation bar ("Profile", "Subscriptions", ...) on the contact page?
Fixed, contact form now uses the main users profile layout