spdy is currently enabled in firefox nightly and you can turn it on in FF 11 and FF 12 through network.http.spdy.enabled in about:config
There is not yet any spdy support for the images on the Akamai CDN that twitter uses, and that's obviously a big part of performance. But still real deployed users of this are Twitter, Google Web, Firefox, Chrome, Silk, node etc.. this really has momentum because it solves the right problems.
Big pieces still left are a popular CDN, open standardization, a http<>spdy gateway like nginx, a stable big standalone server like apache, and support in a load balancing appliance like F5 or citrix. And the wind is blowing the right way on all of those things. This is happening very fast.