One of the better jailbreak apps is WeatherIcon, available in Cydia. This app allows two things: a temperature display in the status bar, and live updates/changes to your designated weather app icon (Apple’s native weather app, The Weather Channel, Weatherbug, etc...) that reflect current weather conditions. This way you can remember it’s a dapper -14 degrees out without even unlocking your phone.
This is a clean and simple app. The temperature display in the status bar is unobtrusive (though there is an option to include a small image of current weather that takes up a little more space) and the various icon graphics for current weather are hi-res for iPhone 4 and well, accurate. Is it raining outside? Why yes, my iPhone weather icon tells me it is!