If the Network Monitor is enabled in Flash Builder, any HTTPService request will return an HTTP 502 error associated with port localhost:37813 on the Google Chrome and Firefox browsers. IE 8 works fine. It does not matter if the application is run via debugging mode in Flash Builder or independently. The work around is to disable the network monitor.
I don't find the network monitor particulary useful. Other tools such as the free Fiddler tool works much better providing more info about the HTTP request and response.
Logged bug FB-21324.
Fiddler provides more detail information about the response/request header and payload (and various view into, sucha raw/xml/html forms). With the network monitor it hard to get a good view into the payload