I downloaded Windows Live Writer last night. It's definitely superior to writing in a browser window and setup -- generally a challenge with offline blog editors -- was okay (it didn't quite diagnose that I used dasBlog, but once I chose it manually, it worked out well. And configuring the FTP upload of images took me two tries.) Hmmm... Don't use the Date/Time property if you're writing for dasBlog.
I also downloaded the SDK. I had two very specific scenarios in mind: scheduling blog posts (that is, write it now, have it upload at a later time/date) and an hReview microformat template. My take is that the API doesn't make either trivial. For scheduling, what you'd want is to hook the posting event; instead, with the API I think you'd have to create an out-of-process local proxy (so, essentially, no help from the API on that one). On the hReview template, you can use the API to open the editor with existing content (e.g., the hReview tags, which are implemented in CSS), so you'd need to... Stand by...