HttpService with JSON from server - help please

Hi!
I am trying to pass back a JSON object to flex using HttpService. The scenario is like this - using HttpService I am calling an action since i am using Struts 2.0 . This action is supposed to return a string in JSON format. When i am passing back the data in XML format the data is properly displayed with the required hierarchail structure. But when I am passing back the JSON string it is not displaying anything.

This is the call to server
<mx:HTTPService id="srv" url="...... / myaction/getSomeData.action"/>

the srv value is coming as null.

I am a total newbie to flex.

Thanks for any help in advance,
Rohit