Well, on my personal computer I hit the same error message. I can't explain why I was able to download the CS5 installers on my MBP. This laptop was issued to my by Adobe, but it's brand new and I've never even logged onto the network with it yet. I'm certainly not inside the firewall. Perhaps something that IT installed on it gave me keys to the ProDesignTools kingdom.
Sorry for the false lead.
edited: Oh, just saw your latest post, R_N. Glad you got it to work. Now I gotta try logging out and back in on my personal machine!