In case you didn't see it, we just recently published a good paper on building your own BizTalk adapters. The paper on MSDN,
Writing Effective BizTalk Server Adapters, is a fairly thorough look at all the things you'll need to consider when
building a BizTalk adapter using our framework. Of course when you build a new adapter, you won't have to start from scratch since enterprising folks have
built things like the free
BizTalk Adapter Wizard.