Adapter Migration Toolkit is released on 01/21
This little piece is designed to help migrating your BizTalk 2002 components (AICs, custom preprocessors) to BizTalk 2004 by providing shims. It's downloadable at the MS download center.
Also, please, notice the whitepaper on BizTalk 2002 to BizTalk 2004 migration in the same download location -- see the link on the right in "Related resources. It's a self-extracting zip file.