I have the same issue as "jakehh".
I have Pages '09 (version 4.0.4 [614]). My OSX is Lion 10.7.1.
I go into the App Store and click on buy Pages and it says Pages is already installed on my computer! The exact wording is: "We couldn't complete your purchase. Pages is already installed on this computer. Choose Software Update from the Apple menu to check for available updates." I have run "Software Update" a number of times and it says my computer is up-to-date! I am stumped. I can't buy the upgrade to 4.1 from the App Store and cannot upgrade it through the "Software Update" function.
There is nothing different in Pages since my upgrade to Lion from Snow Leopard. The window title cannot be clicked. I have hovered over the title, clicked, double-clicked, etc. Nothing. Although I haven't tried testing this fully in Number or Keynote yet, I get the feeling it will be the same issue because I don't see a fullscreen icon in the top-left corner and clicking on those apps in the App Store results in the same msg from the Store --> "We couldn't complete your purchase."
I have attached some screenshots to help.
Thank you for any light you can shed on this issue.