Working on adding support for Snow Leopard here for a Mac app that I have on the MAS, and the only thing I've found so far that I need to replace is my reliance on NSJSONSerialization.
So what's a good library for JSON parsing under Snow Leopard?
I ran into the same problem sometime ago, there are lots of parsers out there, but my favourite (performance wise) is JSONKit
Only downside is it's not ARC compatible, but you can either Disable ARC for the JSONKit Source file OR build it as a Universal Framework, You'll find that in one of the pull requests.