International Travel Weather Calculator developer has released two different apps which show instant weather update on the home screen of iPhone using app widget without opening the app. There are two apps for Celsius and Fahrenheit respectively available in iTunes.
Fahrenheit is backed by an online web server that uses Push Technology to push the current temperature of any location to your iDevice. This makes it possible to always automatically have the up-to-date temperature displayed in the red badge at the top-right of the Fahrenheit icon on your Home Screen. Fahrenheit supports all iOS Devices: iPhone 4, iPhone 3Gs/3G, iPad 2, iPad and all iPod Touch devices!
It is not possible to display the temperature in Celsius. You have to download the Celsius application instead.
You can even share your current weather status with your friends via Facebook and Twitter.
Download Celsius App for $0.99
Download Fahrenheit app for $0.99