If you bought one from Apple just after Snow Leopard was released it would have been pre-loaded with Leopard with a Snow Leopard drop-in DVD in the box as well as the Leopard restore discs. I would imagine the old stock held by Apple will have now cleared and you would get only Snow Leopard in the box.
You could always try other retailers to see if they have any old stock. You would probably have to order the up-to-date Snow Leopard upgrade from Apple if you go for this option.
SL doesn't require leo to be preinstalled , if thats what you mean, so there is no reason to have leo in the box. Still, if you did it would be tied to that specific model type anyway. Only the retail disks would be universal.
I bought my MBP at BestBuy on September 9th and it came with SL installed. It's been long enough to where they should all come with SL now.
As an experiment, I tried installing Leopard on my MBP and it worked ok. The keyboard lights would only come on after I installed all available updates.
The touchpad multi finger gestures were not available. Just the basic one finger click and the two finger right click. The options were still there for all the other gestures, they just would not work. Apparently these are missing from the basic Leopard install DVD unless the MBP originally came with Leopard.