Update:

I can see in the response when doing a query this (Find .NET API Version: 8.10.0.1509):

{"$id":"1","status":"ok","hits":[{"$id":"2","title":"<title>","excerpt":"<text>","url":"<url>","typeName":"Page","hitTypeName":null,"section":null,"subsection":null,"authors":null,"publishDate":"2012-11-28T10:26:00+01:00","updateDate":"2014-08-28T09:08:15+02:00","imageUri":null,"geoLocation":null,"filename":null,"fileExtension":null,"originalObjectType":"<type>","metaData":null},{"$id":"3","title":"...... <next 7 results>}],"total":8}

However on a version that this works (Find .NET API Version: 8.7.0.1103) I can see this in the response after a query:

{"status":"ok","hits":[{"Title":"<title>","Excerpt":"<text>","Url":"/<url>/","TypeName":"Page","HitTypeName":null,"Section":null,"Subsection":null,"Authors":null,"PublishDate":"2011-08-23T12:51:00+02:00","UpdateDate":"2011-08-23T12:51:53.333+02:00","ImageUri":null,"GeoLocation":null,"Filename":null,"FileExtension":null,"OriginalObjectType":"<type>","MetaData":null},{"Title":"... next 37 restults}],"total":38}

As you can see there is an "$id" missing in the one that work.