The apps purchased from App World are on a separate system than those found everywhere else. It's just like if you bought something in a best buy and went to walmart to exchange it. Walmart would have no clue that you bought it since it wasn't in their system. If you should see a sale on mobihand, you more than likely won't see it on App World unless the developer changes the price for the app but then that takes a while to process.
Posted from my CrackBerry at wapforums.crackberry.com