OS X Lion - download once and install on multiple computers
It will be, as announced on WWDC 2011 keynote.
It will be delivered via the App Store for $29, and you can use it to update your other authorized Macs as well.
I'm not to keen with the details. Sorry.

Link to the keynote:
http://events.apple.com.edgesuite.net/11piubpwiqubf06/event/