It's too late to edit my own post so I'll just reply again. I went ahead and started work on the Weather Underground API. It generally works so far, and will automatically use the WU API if a WU API key is provided.
I don't see why not, if there is interest. I really just refactored it to work with my project without jQuery. I'll take a look, maybe make the choice of API selectable.
Work in progress:
https://github.com/a12k/reallySimpleWeather/tree/wundergroun...