I didn't find a tutorial, but it seems like writing a port wouldn't be so hard. It is a Java application, and is based off existing standards (HMAC SHA1).
See this page for details on the non-GUI guts:
- http://blog.jcuff.net/2011/02/cli-java-based-google-authenticator.html
And see these pages for info on porting, and an existing (unofficial) Silverlight port:
- http://code.google.com/p/google-authenticator/issues/detail?id=49
- http://code.google.com/p/g-authenticator-wp7/