The problem is that Twitter broke links to tweet pages by using the #! URL structure. Now you can't load a tweet page without loading the entire app. That's bad.
It's easy enough to use pushState where available and fall back on #!
Or just failover to full page refreshes, like we do at gdgt.
Too bad for IE users. They've said they're redoing it to given users of modern browsers the best experience, and if you use a crap browser you just get full page loads. /#!'s are getting kicked to the curb