Until recently, I labeled myself BizTalk-curious but more accurately it would have been BizTalk-clueless (and...curious). I ordered "Foundations of BizTalk Server 2006" but haven't had a chance to tear into it yet.
Serendipitously, (now there's a word you didn't expect to read here...) my firm just hosted BizTalk 101 for a client. As a Microsoft partner, Microsoft was good enough to deliver a 3-day, online introduction training. Vishal Arora, the instructor performed brilliantly.
I'm certainly far from expert status but with this training, I at least can eliminate my BizTalk-clueless moniker. If you're a partner and want to get into BizTalk, this training is a great place to start. I recommend leveraging it in conjunction with a client.