So - what weather APIs already exist?
For current weather and short range forecasts there are several to choose from -
Yahoo also has one for forecasts http://developer.yahoo.com/weather/
These are nice, but I couldn't make worthwhile use of them in my work, so maybe there's a niche still remaining for climate and historical weather records. An API for all of that would be a big project. So, small moves...
How about a map of all WMO recognised observing stations from WMO Publication No. 9, Volume A?
Here you go -
As I've used Google's Fusion Tables getting KML is easy too - https://www.google.com/fusiontables/exporttable?query=select+col2+from+2146249+&o=kmllink&g=col2
Next small move? Getting CLIMAT reports from CRUTEM3 into a table.