MT3.3 has a new field for tags, and a lot of new tagging functionality. If you have been using the keywords field for pseudo tagging purposes, you can easily populate your new tag field with your keywords, without having to go back and re-enter every one.
Plugin developer Hirotaka Ogawa has written a simple script - mt-keywords2tags that will populate the tag field of your entries with any keywords that are already in the keyword field for those entries.
Just upload the script to your MT directory, the same place you have the rest of your mt cgi scripts (mt.cgi, mt-search.cgi, etc.). Set the permissions of the script to 755. Then point your browser to the cgi file. After the script runs, delete it from your server. Many thanks to Hirotaka for making this so easy for us.