Can I update apps that I've already purchased through the Mac App Store?
From Daring Fireball:


  Panic on the Mac App Store
  
  Cabel Sasser on what it’s like for existing
  Mac apps that are now in the App
  Store:
  
  
    PS:  If you’re a current customer,
    there’s no way to convert a previous
    purchase into a Mac App Store purchase
    — that requires a re-buy. But
    remember, what you have now will
    continue to work just fine.
    
    UPDATE: The Mac App Store may show
    software bought from us previously as
    “Installed”, even though they’re two
    different licenses. You will not get
    Mac App Store auto-updates unless you
    purchase from the Mac App Store. To
    re-enable the “Purchase” button in the
    Mac App Store, just drag the app to
    the trash. Your preferences/sites will
    not be affected.
  
  
  In other words, for apps that use the
  same bundle ID for the both the Mac
  App Store and non-App Store versions,
  the Mac App Store will not install
  updates to already-installed non-App
  Store versions, but it will recognize
  them as being already installed. If
  you want to get updates via the Mac
  App Store, you’ll have to re-buy the
  app through the App Store.