Dave Goldman from the Exchange Server Team has bloged about OAB V4 which ships with Exchange 2003 SP2. Most of the new features found in OAB V4 have been implemented to remove common OAB download problems.
It should be noted that when Exchange 2003 SP2 is installed OAB V4 will be available but your Outlook clients will not be able to make use of them until Office/Outlook 2003 SP2 is installed.
You can read the full post on the Exchange Server Team Blog, "You Had Me At EHLO" here