I've just discovered the clearoutside.com website which forecasts hourly cloud cover for any location up to 7 days in advance. It also has daylight and moonlight hours, ISS passover times and detailed weather forecast for each hour.
They've just released an iPhone app and are in the process in developing an android app.
I don't have an iphone so can't comment on how good the app is but the website looks good. It uses data from 7 sources so it should be more accurate than services that use one source.