My first response to this was, what problem would it solve?
Post-via-email came in handy occasionally on Tumblr and Flickr when I was using a Blackberry. There were no BB apps for those services, and the browser was way too cumbersome.
ThoughtStreams's design works very well on modern mobile browsers. I'm hard-pressed to think of a scenario where using an email client would give you a better experience than just using the browser on an iOS, Android or Windows Phone device (or any desktop environment). Legacy phone users would probably benefit the most.
On the other hand, post-via-email could give TS something like an app experience when combined with Drafts or something similar.