I've amended my MT 4.21upgrade instructions to include revising the link to your MT installation in your Typekey profile, if you use Typekey. When doing a fresh install, the Typekey token will still be pointed at the old install and Typekey authentication won't work. Make sure you change it by logging into Typekey and editing your account preferences.
Any other services that interact with your MT installation (like Flickr's email to blog feature) will need to be updated as well.
