I think what "galfert" is suggesting is using the WU API as your protocol, not to literally connect to the WeatherUnderground. Unfortunately, it's a pretty crappy protocol: it uses an HTTP GET command to do what is really a POST, and offers no way of getting the data back out.
Yes that is what I was saying, use the API as the protocol without needing to actually using Weather Underground. I was thinking also if the protocol was used it could also be used to pull data also from Weather Underground (with the requested JSON key). This way you could add flexibility to the application of where the data comes from.
I have though read that WU will soon be making some changes to their API. Surely if it has shortcomings then they should be corrected, but it still serves as a good starting point for a protocol to use.
Tom,
When you say the WU API protocol offers no way to get data out, I think you are referring to a bulk historical download. Because it is capable of getting data out that is Live Data.