As of 2018, Google no longer supports special crawling for hash bang AJAX.
I would choose to use push state rather than hash bang AJAX URLs now. Google's Matt Cutts just said this about push state:
A correcly implemented site that uses push state typically doesn't need any extra support enabled for us to crawl it.
It should be much easier for other crawlers that might not support hash bang to deal with a push state site.
According to a web page that no longer exists, Bing does now support hash bang crawling. I haven't been able to find information about the meta fragment support for either Bing or Yahoo. My guess is that if they did support it at some point, they have also stopped supporting it.