Following up on the concept of mindshare and market domination via APIs, I finally found a common XML-RPC API for weblogs, which seems relatively well-designed. There's also a completely rearchitected API that's incompatible, which of course I like slightly better. Naturally the existing API doesn't support versioning of the messages being received. I'm of the belief that the API in general would be improved by a tool-neutral name, a higher level of abstraction and extensibility, and a design more informed by some of the concepts (but not necessarily the weighty implementation) of WebDAV.
Show-stopper omissions in the API as proposed are a lack of time-based functions, including get/set post or comment time, and editing or deleting posts/comments by time or date range. This seems to be more fleshed out in the Wasabii spec. More discussion, as ever, in the relevant Yahoo Group.