I like that too. markdown@posterous.com -- that kind of change is going to take a little more than an hour's worth of work, though... might have to wait til morning.
Incidentally, there's another huge vote for Markdown from this corner---for me it kind of takes posterous from the toy column to the essential column. Which is not meant as a slight to posterous; I think it's the most elegant blogging system out there, bar none. But a lack of some kind of structured text means that I'd be having to hand-code html in a mail window. And I promised myself I'm never going back there... NEVER.
