Check to make sure IIS is properly configured to serve up your WCF services (see MSDN).  Next check out Rick Strahl's blog, specifically this link, this one, and this one.  He had some great presentations at DevConnections last year in Vegas.  That was the turning point for me when I decided to start using jQuery and been very pleased since then.

Good luck!

Any chance you have the UrlScan enabled on your server?  I came across this post by Scott Hanselman last week or so which just popped in my mind right now as a possible issue with your WCF service. 

On that same note, there was something Rick Strahl mentioned at the conference about "seemingly" empty data posts and how he had to hack it and force at least 2 bytes of data to be posted even when they weren't necessary.  I wish I could remember *exactly* what he said about it but I can see that your data is definitely empty ( data: { } ).  I have to run right now otherwise I'd look for what he said about "empty" posts with WCF and the workaround but suffice it to say that definitely is an issue that he brought up and possibly the issue you are running into.