Hi All (and thanks for the email pivic),
It seems that there are a few things going on here:
1. Some people don't have the core PHP JSON extension installed/enabled, so they require an additional library to be loaded for certain things (e.g. IntenseDebate) to work.
2. WP 2.9 introduced a new core JSON library, which will override the one included in IntenseDebate. That should theoretically be fine but
3. 2.9's JSON library also had a bug in it, where it was sending unnecessary headers to the browser.
This problem only seems to manifest itself on a server where the JSON extensions are not enabled, and where you're running WP 2.9.
The ideal solution is to enable JSON on your server, but unless you have full control over your server, that's probably not possible. @pivic you said that you tried getting the class-json.php from RC1 -- was that specifically for wp-includes/class-json.php?
Can other folks please try replacing their WP file at wp-includes/class-json.php with this one: http://core.trac.wordpress.org/browser/trunk/wp-includes/class-json.php (use the links down the very bottom to download).
If that doesn't work, please let me know what version of PHP you're using on your server so that I can look into this a bit more, as I'm not actually seeing this error on any of my installs so it's tricky to debug.
Thanks for your help and patience!
Beau (from IntenseDebate)