Opening the weather application every time to check the current weather has always been a big pain. I've always wanted something to display live weather on my home screen, though the Weather Widget did it, it occupied a lot of space on the home screen. But now, there's a utility on Cydia which lets your weather icon show the current weather. Though this can be battery consuming it's worth a try.
First of all, WinterBoard (which you can get from Cydia) is a must to install this tweak. Then, you can simply download this little hack named WeatherIcon from Cydia (in the MMi source) and activate it in the WinterBoard application (make sure you move it above any theme you have enabled on the phone so that it replaces the theme's weather icon).