Strictly, yes. It is theft, since you are stealing the bandwidth they have paid for.
I have heard arguments that if somebody leaves their wireless network open, then they are inviting you to use it. Leaving your front door open may be silly, but it is not an invitation to anyone to take your contents.
It is even illegal to find an open network and leave a message warning them that it is open!
There are a number of businesses that offer free wireless connection to encourage you to use their premises. Unfortunately, even more are short-sighted and want to charge ridiculous fees for that.