With OS X restore disks it's possible to bypass the machine check. This is particularly handy for Intel Tiger machines as the retail version was PowerPC only. With earlier versions of OS X I'm not sure if the check was absent or just contained a wider range of supported machines. I know I used the Jaguar discs from my iBook with my iMac and possibly a G3 or G4 as well.

For Snow I used this to unlock a Sandybridge MBP restore disc. Came in handy when I built my SB-E Hackintosh.