Pat, the json structure looks very familiar to a problem i described here - The answer for me was to treat the json representation as a Dictionary<TKey, TValue>, even though there was only 1 entry.
If I am correct your key is of type string and the value of a List<T> where T represents the class 'TheUser'
HTH
PS - if you want better serialisation perf check out using Silverlight Serializer, you'll need to build a WP7 version, Shameless plug - I wrote a blog post about this