I don't know, it's like buses — you wait years for a decent writing environment to come along, and then several are in development at the same time. Matt Gemmell at Irate Scotsman has proposed a writing editor with the provisional name of 'Treatise'. There's a UI mock-up here. It seems to have all the proposed features I was looking for, as did the project Nathan mentioned that he was thinking of pursuing (in the comments on the entry I wrote in July on this subject). It will be really interesting to see what develops. Sorry — no pun intended there.