I'd be happy to share the usage stats of WP.com's post-by-email feature if Posterous would be willing to do the same.
I'm sure a lot of people would be interested in those numbers. I always assumed a tiny percentage of the WP userbase would employ that particular feature since it's kinda geeky... but definitely would like to know because it does have may implications for other webapp owners.
PS: Congrats on the 3.0 release :) Will be upgrading a couple of sites tomorrow.