Note: This question is out of date, please see the other answers.
This does not appear to be supported.
However, Microsoft has been made aware of this issue and there is a work-around:
You can get the desired effect by
omitting the Query string from the
UriTemplate on your WebGet or
WebInvoke attribute, and using
WebOperationContext.Current.IncomingRequest.UriTemplateMatch.QueryParameters
from within your handlers to inspect,
set defaults, etc. on the query
parameters.
https://connect.microsoft.com/VisualStudio/feedback/details/451296/