A new BizTalk Server 2006 whitepaper has been posted on the subject of developing a transactional BizTalk Adapter using the Microsoft Base Adapter Classes.
You can find the paper here:
http://download.microsoft.com/download/3/7/6/376a6f6c-8c97-4ab5-9d5a-416c76793fbb/Transact_Adapter.doc