What is the preferred method of altering a layer's symbology dynamically? My web app consumes a map service via the REST API, but I don't mind using the SOAP API or ArcObjects (.NET).
The REST API is stateless, so you can't change the symbology via a connection to a RESTful service (although you could use a graphics layer to dynamically extract the features you want to display differently).
You'll have better luck using the SOAP API (via the Web ADF or simply connecting to the service via SOAP manually) and altering the symbology there. More information about this is available here: http://forums.esri.com/Thread.asp?c=158&f=2421&t=266974