System.Json namespace missing from Windows Phone 7
Add a reference to System.ServiceModel.Web as well as the reference to System.Runtime.Serialization and it should work. System.ServiceModel.Web appears to add .JSON to the Serialization namespace.
You could also try using the DataContractJsonSerializer, which is definitely present in the MIX RC tools http://msdn.microsoft.com/en-us/library/system.runtime.serialization.json.datacontractjsonserializer.aspx.
Seems that JSON is supported in Windows Phone 7 in the System.Runtime.Serialization.Json namespace, hopefully this will be useful, even though this question is a month old!Information from the TechDays in London (via Twitter) was not there personally.
Just an update is that the namespace and class I quoted here was incorrect so here is the correct namespace and class:
System.Servicemodel.Web and the Class is DataContractJsonSerializer for JSON Serialisation and Deserialisation on Windows Phone 7 - sorry about the inaccurate reply I have corrected after a comment posted here.