I'm seriously, seriously impressed. Less than 48 hours after pinging Scott Woodgate about the mapper bug mentioned below and sending over a repro, he and a colleague have confirmed that it's a bug and registered it for a "likely" fix in SP1.
Thank you so much for the rapid response - being a BizTalk developer really makes you feel like part of a community, and having such a direct and effective interaction like this can only add to that.