You need to turn on sync in MobileMe prefs in System Prefs, (select only iCal if you only want to sync the calendar) and turn on sync on the phone (ie push).
They will both sync with the MobleMe server. So, yes, when you add data your wife's computer it will sync with the phone.
If the computer preferences is set to automatic it should happen within a few minutes).
Word of caution. When you tell the phone to turn on push for the first time it will warn you it will delete the existing data in the phones Cal. So it best sync in iTunes first, thus there is the same data in both places to start with.