I purchased my iMac a little after Leopard was released. It came installed with Tiger, but included a Leopard upgrade DVD. A few days ago, on a whim, I stuck the DVD into my Powerbook, which had Tiger installed, and managed to successfully install Leopard. It seems to be completely functional and stable. My impression was that the drop-in DVDs worked only for the model they come with, but in my situation, that is clearly not the case. Have others been able to do this? Also, as I speak, I am "downgrading" back to Tiger on my PB. There are still a few Classic apps I use, and I feel bad breaking the EULA.