Adobe AIR: Handling JSON objects from server
You can find a JSON parser written in JavaScript here (source code here). You can also use the as3corelib JSON parser from JavaScript, there's a description of how to access ActionScript libraries from JavaScript here.
The current AIR release (v2.5) bundles a newer WebKit that has native JSON support, via JSON.stringify() and JSON.parse().
Have you looked at as3corelib? It appears to provide an AS3 parser for JSON data, and my hope would be that it doesn't rely upon eval (eval tends to be bad for security as you noted). There are similar libs for Javascript as well, and they tend to be the preferred way to parse json due to the security implications of calling eval on (potentially) evil data.