I maintain a Firefox extension called Firegoose that (among other things) works with data embedded in HTML pages. We do this using microformats, embedded XML, and even, if we have to, nasty screen scraping. I'm not too religious about the format of the data - just give me something I can parse.
So, it occurred to me to wonder why JSON wasn't more popular in this niche. Easily parsable, legal in a browser, scriptable from within the page - why not? Of course people are, as usual, miles ahead of me here:
Strangely, activity around this topic seems to have surfaced briefly and disappeared sometime in mid 2007.