Sorry to dredge up a week old thread, but I was wondering the same thing as the OP and this came up from google.
I'm a die-hard Mac user since the old days but I think they've really ****ed the goat on this one. People want to buy Leopard because 10.6 won't install on PowerPC machines. There are still a LOT of PowerPC machines out there, perfectly capable of running 10.5. Apple could be selling all their old Leopard discs instead of burning them in a huge bonfire (as I'm sure they're doing) and still making money. It's not like anyone is going "oh, well, I guess if they don't sell 10.5 anymore I'll just buy a new computer," even though that's exactly what Apple is thinking.
This has gotta be the ass-ugly brainchild of some new exec somewhere at Apple, because it's not limited to the 10.5 issue; one other example jumps to mind immediately: Mac Pro AirPort cards. They still SELL the Mac Pro with the card as an OPTION, which means you can get them WITHOUT the card, but they've deliberately pulled the card from the store (online and retail). You can't get them anymore through retail outlets. I talked to a tech at Apple about buying a wireless card and the poor guy was stumped that they didn't sell them anymore. He was searching on Amazon trying to find me a wireless card.
Anyway, it's retarded. I say pirate the hell out of 10.5 and bypass any and all licensing restrictions if you can. Even if they cared, they wouldn't take you to court because all of this deliberate upgrade-forcing would come out.