well, the calendar one does it because it's running from data on the iPhone itself...if the weather had to update all the time to keep matching, the AT&T network would get choked very quickly...it's a great idea, but unfortunately not practical. Too bad though, it would be cool...
Nevermind, read the question wrong. I missed the word "icon" and to spare myself embarassment, I'll just delete. 😉 I agree with the OP, it would be cool.
There's not much difference between constantly checking for email and updating the weather.
Lots of Windows Mobile phones have weather and forecasts continuously displayed on their homepage. Just glance down and see it without having to go into a dedicated app unless you want to.
One weather plugin I use lets me assign up to six refresh times a day. Others let you have even more control. I don't notice any battery difference.
Besides, isn't every iPhone always next to a WiFi hotspot? 😉