Update: You can see the latest benchmarks at https://github.com/samsoffes/json-benchmarks. The results change quite a bit, so I'd recommend just running the code and seeing who wins.
I wrote a post awhile ago about JSON benchmarks. I was telling a friend he should use JSON Framework based on my old benchmark post. He asked if I had run them again recently, so I figured I'd run them again.
I updated my test app and added a new library called YAJL based on a C library. My results were very similar to before. This time I tested it on an iPad and iPod Touch.
On both devices, they ranked Apple JSON, YAJL, JSON Framework, and TouchJSON. You can read the detailed results on GitHub.
In conclusion, it looks like YAJL is the new one to use. Again, feel free to check out my code on GitHub.