Ajax.net 3.5 JSON and WCF Serialization

 I'm trying some WCF ajax.net 3.5 out for the first time, and I'm having some difficulties.  I have a WCF Service which I have connected to in my aspx page using <asp:ScriptManager>.  I have a custom object which has been decorated as a [DataContract].  I'm calling the object from javascript successfully, I can see the JSON response in FireBug, but Sys.Serialization.JavaScriptSerializer.deserialize() chokes on it.  I'm sure I'm making a noob mistake, but I used up all my patience getting the Web.Config values right for running the WCF service :)
Here's a mockup of what I'm doing.

// The custom object

namespace my.namespacehere
{
[DataContract]
public class UserAccountData : IComparible
{
[DataMember]
public Guid AccountGuid { get; set; }

[DataMember]
public string Email { get; ...