Look to the lease. The rights and obligations of both landlord and tenant are spelled out in detail in that document. If the lease agreement says that you have until 10/15/10 and there is no provision that you have to leave if the owner wishes to move back in, I would think that that is the beginning, the middle, and the end of the story.
Please note that I am a New York attorney and cannot advise you as to the application of Washington state law. For that reason, you may wish to consult a local attorney.
Good luck to you.
Your landlord needs a court order to remove you from the premises under RCW 59.18.290. In addition, if they breach the lease they need to pay your actual costs resulting from their breach, such as costs incurred as a result of having to move early, if any.
As a practical matter, though it is annoying to be uprooted, the easiest thing for you to do would be to negotiate a plan with them to help you move and make sure you're not overpaying for your last month. Keep everything in writing, memorialize phone conversations with confirmation letters. Remember that small claims court is cheap and handles this stuff routinely. And remember that again when it comes to your security deposit. Good luck.