Jim Plush has a new post today with his look at the talk surrounding the introduction of JSON functionality into the PHP core.
I see JSON becoming the dominant player in the AJAX world in the upcoming future. It's ease of entry, especially if bundled with PHP will make it a no brainer for AJAX or AJAJ applications.
Rasmus made a posting on the php internals list regarding rolling Omar Kilani's JSON Extension which serializes/deserializes datatypes into Javascript Object Notation and vice versa. Since this is done in C space is MUCH faster (85 times from Omar's perf testing) than the existing PHP JSON classes. http://www.aurore.net/projects/php-json/
The suggestion is quoted in the post, and it seems that Rasmus only suggests that the extension be included in PHP. As far as I'm aware of, though, there hasn't been any action to pursue it...