If you ran one of the beta releases of Mac OS X Lion, you probably ran into a problem when trying to upgrade to the final release of OS X Lion.
When I tried to upgrade one of my dev-machines, I was prompted with an error saying “A newer version of this app is already installed on this computer”
Of course, you know that isn’t the case, as OS X Lion, just came out.
After some digging, I found the solution. It turns out that all you need to do is to hold the option key (⌥) while pressing install. That will force the Lion to install.