I just noticed that a new whitepaper was published recently by the BizTalk team called "Writing Effective BizTalk Server Adapters" [through luke nyswonger]. Strongly recommended if you are writing custom adapters, as the whitepaper explains the not-so-obvious sides of how the messaging engine works and how the different types of adapters should behave. It also contains valuable tips on how to ensure your adapters work reliably and with good performance.