Yeah, I love tumblr for a lot of things, but it's impossible to have any kind of discussion there. (Though I actually prefer the new non-nested reblogging style, which at least makes it less difficult to follow a conversation if it does happen.)

Anyway, the startup is PillowFort (http://pillowfort-io.tumblr.com/); unless something has changed, they plan to launch a kickstarter in January and open up a beta in February.