I’ve learnt some fairly painful lessons today on my Mac, thought I’d share them..
(1) Snow Leopard discs are tied to the model of Mac they are sold with
(2) wiping a Macs hard disc doesn’t necessarily remove all vestiges of the previously installed OS.
(3) scratched cd and dvd discs are %##}{#%%!!!
The key was to reset the PRAM before using a lion DVD:
https://discussions.apple.com/thread/3207417?start=0&tstart=0