Whatever happened to Response.Headers.Add from the friendly web code I'm so used to... Oh well, after scratching around for ages I finally found how:
var prop = new HttpResponseMessageProperty(); prop.Headers.Add("ApiVersion", "v1.0"); OperationContext.Current.OutgoingMessageProperties.Add(HttpResponseMessageProperty.Name, prop);
I know you're kicking yourself, right? "That's so obvious..." Ridiculous - I'm starting to thing WCF REST wasn't made with the developer in mind.
Update:Actually, there is an easier way I found. Since this is in a REST service it is possible to use WebOperationContext which seems a lot more intuitive and concise: