Backbone encourages pushState() if you can swing it -- you'll need to implement every client-side route as a server-side page as well ... so it's not possible if you're hosted in an inflexible environment. Backbone falls back to hashes for browsers that don't support the API, and transparently converts URLs between the two.

But regular hashes ("#") are fine for internal single-page-apps that don't need google indexing -- of which the majority of Backbone projects probably are.

It's just the hashbang "#!" that's a particular pet peeve of mine ;)