Tried Lion on my Air, watched as the CPU overheated and then torched the logic board and SSDs, and had Apple replace everything -- then reverted to Snow Leopard immediately!
Thank goodness I never tried that with my Pro.
Seems to me, six months later, that Lion was a good excuse for desperate workarounds. Still waiting for all the positive news about how much it actually makes computing better. Nada.
What's not entirely clear from the wording of this update is whether or not it's critical for an in-situ upgrade from 10.6.8 > 10.7 on an individual Mac, or is just for users physically moving to a Lion Mac from a Snow Leopard Mac.