The YAHOO.Tools package now includes a version of Douglas Crockford's JSON.js file that doesn't mess with the object prototypes.
You can see an example of the JSON parser here.
This function attempts to identify the core browsers through object detection. However, a developer may trash this detection by adding "functionality" to a page.
Go ahead, change your User Agent string. This should still work 8-)
This function uses the User Agent string to determine it's information. It will be fooled by a User Agent switcher