Check out Charles Young’s post “Dublin and BizTalk Server - What's the difference?”. It’s well worth the read, for all BizTalk developers, and a very interesting analysis. I’ve been to some of the events Charles mentions, where there was some discussion about this new application server and its relationship to BizTalk Server, and this a curious solution Microsoft has found. I do have a complaint, however: I do feel BizTalk Server could be improved in several areas, and the last two versions (R2 and the planned 2009) have been somewhat lacking in this aspect. Things like the Orchestration Designer, BAM, BRE and even the mapped could clearly be improved (and don’t get me started in Usability), and low-latency support added, but Microsoft has not focused the evolution of the product in these aspects, which I regret.
Anyway, a highly recommended read.