Is it legal to use 2 authorized computers in different households? I would like to share my music with my girlfriend. Can we share a single apple ID? 

What does the following really mean?: "Apple is the provider of the iTunes Service, which permits you to purchase or rent digital content ("iTunes Products") for end user use only under the terms and conditions set forth in this Agreement." (From Itunes' Terms of Use)

Does it mean that the person who owns the account and buys the music is the only one who can be "end user"? Because if I share a single account with my girlfriend, she (the account's owner) would have the right to be end user but could she legaly give me part of that right too? At least for one of the 5 computers she can authorize?

(note for future anwswers: we are NOT in the same household)
This is covered under the USAGE RULES section of the iTunes Terms of Service, which reads, in part:


  USAGE RULES
  
  (i) You shall be authorized to use iTunes Products only for personal,
  noncommercial use.
  
  (ii) You shall be authorized to use iTunes Products on five
  iTunes-authorized devices at any time, except for Content Rentals (see
  below).
  
  (iii) You shall be able to store iTunes Products from up to five
  different Accounts at a time on compatible devices, provided that each
  iPhone may sync tone iTunes Products with only a single
  iTunes-authorized device at a time, and syncing an iPhone with a
  different iTunes-authorized device will cause tone iTunes Products
  stored on that iPhone to be erased.


While I'm not a lawyer and this is not legal advice, I do not see where it limits usage to one person, or declares that all these devices and authorized computers must be owned or operated by only one person.

I suggest you carefully read the entire legal agreement if you want to resolve this for yourself.