"Upgraders somehow are supposed to know in advance to save the Installation disk image for a bootable back-up."
Clarification: When I downloaded LION a couple days after it hit the market, I was ASKED during the process if I wanted to make a backup disc copy. I selected YES, inserted a disc, and continued through the procedures.
If one chooses to "skip this step" when asked, the chance to do so at that time is lost.
P.S. A DMG was also part of that initial download. Although I have not tried to burn a disc from that DMG - don't need a second one - I presume that it is possible.
