They were switching from hashbang routing to pushstate, not from client to serverside rendering. Although they might to pre-rendering for IE, which will get support of pushstate only in version 10.

http://storify.com/timhaines/hashbang-conversation

Hopefully they are going with pushState with a fall-back to server rendering for all user agents that don't support pushState.

That's a compromise I can get behind. And it's pretty much progressive enhancement...

I thought they were simply changing over to compiling their mustache templates on the server to speed up the process.