Can I use the new Lion OS X license on two machines, or do I have to buy one for each?
Like everything else you purchase in the Mac App Store, you can install Lion on all of your Macs after buying it once. There is no activation process or DRM.

Here's the relevant excerpt from Lion's license agreement (emphasis mine):


  If you obtained a license for the Apple Software from the Mac App
  Store, then subject to the terms and conditions of this License and as
  permitted by the Mac App Store Usage Rules set forth in the App Store
  Terms and Conditions (http://www.apple.com/legal/itunes/ww/) ("Usage
  Rules"), you are granted a limited, non-transferable, non-exclusive
  license:
  
  (i) to download, install, use and run for personal, non-commercial
  use, one (1) copy of the Apple Software directly on each Apple-branded
  computer running Mac OS X Snow Leopard or Mac OS X Snow Leopard Server
  ("Mac Computer") that you own or control;