purist fanboy in me has had me pshaw'ing at JSON. But, now that the recent JSON release from Yahoo! reminded me of its existence and I gave it a shot myself, I have to admit that it's pretty hot—if only for the cross-domain bridging capabilities and the no-fuss parsing.
Although, I do worry about running into a poisoned payload someday that raids my cookie jar.
JSON is currently the big Hot Thing at work. It's very good for some things, but IMO it's not nearly as robust as XML as far as generic interchange goes. Its big drawback is that it's not nearly as flexible, and since elements are either unordered or stored in an array, you have to agree upon the actual structure of the document before you send it across the wire (which isn't so much the case with XML where often all you care about is the nesting order which you can handle with XPath or similar).
It's GREAT for AJAX though, as long as you can trust the server of course.
I thought AHAH was the current big thing. Just send HTML to the client and use