I've posted this myself at least once and others have said it too, but machines almost always run the OS they shipped with, and that OS only. So if your MacBook ships with 10.5.2, not only will it not run Tiger, it won't even run 10.5.1 or 10.5. It's purely a matter of Apple only writing drivers for new hardware (the X3100, Penyrn-ness) with Leopard compatibility in mind. Either I read on here (or been told— probably both) that people buying Mac Pros that shipped with Leopard could jump up and down to Apple and get a set of compatible Tiger discs for those machines. I know for a fact a large company tried this with a set of previous gen MacBooks (the 2.0/2.2 SR ones) and were told in no uncertain terms to get lost. I suspect the Mac Pro situation was a sop to pro customers with specialised software (Logic plugins, FCP hardware, etc), but it's still silly for them. Not that they're not busy at the moment, mind!