There are several WinterBoard based weather themes that can be "tricked" into rendering the weather on the statusbar with several caveats:

1. Only shows on home screen, not while applications are running.
2. Doesn't shift to fit when other statusbar icons show (StatusNotifier, music play, etc).
3. Requires Winterboard which can be a resource hog.

Maybe this is just an add-on to StatusNotifier, but the idea would be that it could be configured to load the weather periodically and show a small graphic for the current conditions and the current temperature. Even better would be if you could configure it to show a static location or where ever you are at the time it updates so the weather information is pertinent to your current location.