I have installed three copies of Lion via the App Store and their DRM requires each to be a separate download. However if you are using the same App Store account you only pay for the first download and the rest are free (up to I believe a total of five installs).
You can actually burn DVD install media, if you like.
Download the Lion package. Right-click the downloaded file and choose "Show Package Contents" from the downloaded installer. You'll see a folder called Contents. Open it. Inside you'll see another folder called Shared Support. Open that.
In the Shared Support folder, you will find a file named "InstallESD.dmg". As the name suggests, this is a complete, bootable disk image for Lion, ready to be burned to DVD with Disk Utility.
Run Disk Utility. Drag the InstallESD.dmg into the Disk Utility Volumes pane on the left-hand side of the window. Click on it and then click Burn. Viola! Your own bootable Lion installer DVD!
