The policies are that it is okay to share accounts with members of the same household.

(by that I would say it was people in the same physical household - i received a temporary ban when I was working away from home and disconnected an account my children were on at the time.. The ip addresses were different and I guess they did see they were physically far apart). EXplaining the situation resulted in reinstatement and confirmation that members of the same household can play the account.

I know there is a lot of hate for blizzard, people calling them greedy etc but I think blizzard are actually very generous - They allow us to create multiple characters across mulitple servers, and provide hours of entertainment for not a lot of money. I couldn't rent 3 DVD's for the price I pay for hours upon hours of entertainment a month (although as a multiboxer I increase the costs without being able to play more than I did before, but I do multiply the possibilities and enjoyment)

It is a business after all, and a business is about making money, but they have a vested interest in keeping their customers happy. (otherwise we would all move to some other game))

In answer to your original question, if you live with your girlfriend then it would be acceptable to let her play your account.

I believe my playing of accounts that resulted in the ban would also be considered within the terms of policy, but I think I fell foul of automatic tracking of ip connections to an account used.