Brilliant post by Wil Shipley on his blog called The Mac App Store Needs Paid Upgrades, well worth a read:

Right now developers selling through the Mac App Store face a lose/lose choice: either provide all major upgrades to existing customers for free (thus losing a quarter of our revenue), or create a “new” product for each major version (creating customer confusion) and charge existing customers full price again (creating customer anger).

Here’s hoping Apple is listening.