Like many Mac users I am in the process of upgrading my Macs to Lion today. With 8 Macs in our house downloading 3.7GBs of installer seems like a huge waste of bandwidth. You only need to purchase the upgrade once for all of the machines you own or control so here's a technique that may help you only download Lion just once:
1) Purchase Lion on one of your Snow Leopard Macs
The download is about 3.76GB so depending on your connection speed it may take a while. You'll need to be current with Software Update if you aren't already.
2) Once the download is complete you will be presented with this screen:
4) Open Finder and navigate to your Applications folder
You should see the installer with the Lion icon titled "Install Mac OS X Lion". Copy that 3.76GB file to portable media device (like a Flash drive or external hard drive).
5) Copy the Installer application to the Applications folder of the Macs you want to upgrade
6) Ensure each of your Macs is running Snow Leopard and current with Software Update
I'm not sure this is completely necessary but better safe than sorry. There have been recent updates to the App Store that enabled people to purchase Lion upgrades; if you don't have those updates I'm not sure if the upgrade will work. If you are still running Leopard on a Mac it will first need to be upgraded to Snow Leopard and updates applied.
7) Backup your Mac before your upgrade
Though the Snow Leopard to Lion upgrade is pretty smooth and few people appear to be reporting problems, having a decent backup (even if it's just Time Machine being completely current) is always prudent.
8) Run the Installer on each of your Macs
The upgrade took about 35 minutes to run on my new MacBook Pro, though your mileage may vary.
I'm putting this out before I've completed the upgrade process on all of my Macs because as soon as you execute the Upgrade the installer is removed from your Mac and will need to be re-downloaded (or copied as I mention above).
If you have any other experience with upgrading multiple Macs to Lion please leave a note in the comments!