Gawker did this a while back. And reverted after their Google rankings took a hefty plunge. That is one very large caveat, that Ajax crawler spec is no magic bullet.

Nick Denton: "Dip in uniques largely because of drop in Google refers. Pageviews (which are driven more by core audience) less affected." -- http://twitter.com/nicknotned/status/61152134929981440

Nick Denton: "Google does not fully support "hashbang" URLs. So we're eliminating them rather than waiting for Mountain View." -- http://twitter.com/nicknotned/status/61465859079671808

Nick Denton: "Yeah, I'd advise against hashbang urls. Will kill search traffic -- even if you abide by Google protocol." -- http://twitter.com/nicknotned/status/62595141927583745