I have the same problem. I have a subscription to weather underground in ical to update daily. It updates to the current forcast with 7 days look-ahead on my mac with ical. On my iPhone or iPad, it is not in sync with ical, I have to turn off calendar under my icloud settings - and then on again to update with the latest info - a simple refresh in the calendar app does not update the weather forcast.
Likely a bug.
A work-around that I have used is...
On IOS 5 devices you can subscribe to a calendar directly.
In the settings... mail, contacts, calendars... add account... select "other"... select "add subscribed calendar"... paste the link from wunderground - and you are good.
You will then have to unselect the iCloud weather subscription (the one that does not update) so you do not have 2 weather entries for each day & you are good to go....
Hope this helps.