Hi Chris,
The most probable case in which you can get new 10.0 formatted compressed geometry through 10.0 REST endpoint is if the underlying network dataset is a 3D one, or somehow server produces output route geometry with measures.
If you are using your own network dataset, please make sure that none of the network dataset source feature classes are 3D aware. If any of them is a 3D feature class, you will have to make them 2D and rebuild the network.
To make sure that NAServer does not return compressed geometry with measures, please make sure tat you pass RouteParameters.outputLines = "esriNAOutputLineTrueShape". Also, to be absolutely sure, open the published map document in ArcMap, open corresponding Route layer's "Analysis Settings" dialog box and check that "Output Shape Type" value is set to "True Shape". Save the document, restart the service, and clean the REST endpoint cache (or restart the web server).
Dmitry