If you have entered a CS5 upgrade serial number then the installer will require a serial number from an earlier qualifying version in order to install the software with a perpetual license.
If you don't have a serial number from an earlier qualifying version (only CS2, CS3 or CS4 qualify for a CS5 upgrade) then you won't be able to install CS5 other than as a 30 day time limited Trial version.
That has always been the case with upgrades.
If you enter a full version serial number then the installer will install the software with a perpetual license since a qualifying earlier version is not required (since it's not an upgrade).
I understand that however I have had Fireworks since it was Macromedia and and have gotten every damn upgrade they have had.
OK, good. So do you not already have a CS2, CS3 or CS4 serial number?