All modern browsers support native JSON encoding/decoding (Internet Explorer 8+, Firefox 3.1+, Safari 4+, and Chrome 3+). Basically, JSON.parse(str) will parse the JSON string in str and return an object, and JSON.stringify(obj) will return the JSON representation of the object obj.
More details on the MDN article.
I know the support is not widespread, but using this method should be a lot faster and safer than eval()ing a string, so I want to use it where it's available. Any idea on support from other browsers?
I didn't say don't use it, I said don't count on it. Definitely check to see if it's available (at this point only IE8 and the few Fx Beta users) and use it if so, but I'm just saying that you shouldn't assume the browser supports it. As of now, those two are the only browsers that support it, and WebKit is working on it right now, so it'll probably be in Google Chrome and Safari sometime soon.
Oh, and on a side note, NEVER eval() JSON strings. Instead, use one of the many JSON parsing libraries available.
@TheXenocide: Good point, but its author probably spent a good chunk of time on that validation code, so I say never eval() JSON strings because you will be reinventing the wheel and you will likely get it wrong.