Native JSON support in iOS?
Updated answer for iOS 5:
JSON support is now native to iOS with NSJSONSerialization, but in terms of performance, it pales in comparison to JSONKit, as John Englehart stated in the JSONKit README:
UPDATE: (2011/12/18) The benchmarks below were performed before Apples NSJSONSerialization was available (as of Mac OS X 10.7 and iOS 5). The obvious question is: Which is faster, NSJSONSerialization or JSONKit? According to this site, JSONKit is faster than NSJSONSerialization. Some quick "back of the envelope" calculations using the numbers reported, JSONKit appears to be approximately 25% to 40% faster than NSJSONSerialization, which is pretty significant.
Here's a blog post which delves into more specific benchmarks: JSON Libraries for iOS Comparison