Actually, the $29 download is the "family pack". Like anything else bought on the app store, you can use the Lion OS upgrade on multiple computers with a single purchase.
MacWorld recommends backing up the download file before installing, however, as the installation will wipe away the download file, and you don't want to have to pull the whole download off the web again for each machine.
I was wondering about that, but couldn’t find any mention of that.. Anyway, it’s no big deal for me to run over to the apple store.. Always fun..