I'm having the same problem here.

Purchased it as an upgrade for Poser 7, after hours of downloading (.......) I finally got the file and installed it. When I open the program it says I must enter my serial number, put in the new one for the Poser 8 Upgrade that I got when I purchased it, but can't do anything.

The box where you have to enter the old serial number is greyed out as well.

Anyone has an idea?