Darron Schall has just checked in some changes and bug fixes to the JSON support in as3corelib. Among other things, Darron has added a strict mode which allows you to specify how strict of a conformance to the JSON spec you want the library to follow.
You can read all of the details over on Darron’s blog. As Darron notes, these are not in the build yet, so if you want to play around with the changes, you have to grab the source.
I will try and do a new build soon with some more JSON changes, as well as some new code contributed from the community.