We're still patiently awaiting the arrival of iCloud. Some are already testing things out, because they have developer accounts. At least one user has discovered what happens when you come close to exceeding your iCloud storage limit (via MacRumors)
Apple will shoot you an email from the "MobileMe Quota Service warning you that you are going over. The note informs you that once your iCloud space is full, devices will not back up while Me.com email addresses will bounce. The user is told to buy additional storage. Also worth noting, is that no official details on storage prices have been released.