Google did this for a few days not too long ago, and got a firestorm of bad press about it. Twitter Search is doing it now and annoying the crap out of me every time I use it. Unless it's done VERY right, it's a really bad user experience.
GMail can get away with using hashtags for navigation because it's an application you're never going to search or bookmark externally. Most plain-old web apps don't have that advantage, and therefore are bad candidates for hash-based navigation.
So yeah, this is tech that's been around for a while, and it's a convenient way to allow the browser back button to work in a single-page app. But I wouldn't recommend using it unless you have a good reason.