My room mate has not signed the lease yet and we have been here for several months, the landlord has not notice or cared. Is this a problem? What implications does it have? Renting an apartment in Washington

The roommate is occupying the premises and paying rent, correct? Then signing the lease is just a formality.

If you have signed a lease and your roommate has not, then you are fully responsible for fulfilling the terms of that lease. If for some reason the two of you break the lease then the landlord would hold you fully responsible and you would have to go after the roommate for reimbursement. Also, your roommate could technically give you 30 days notice and move out at any time without any further responsibility.

Whiffed on that answer. Even if the roommate signed, the lease is probably worded so that you would each be individually responsible for the entire rental.

However, if they didn't sign, then they could claim they are month to month tenants unless they were specifically named on the lease. Then you could claim that they are a party to the lease.