The plane was scheduled to have it's first test flight last month, but testing showed the need to delay that process again. Originally, it was supposed to be in service in time for the 2008 Summer Olympics.
But Delta is apparently optimistic that they will one day in the future be flying the 787-800 in and out of the S-Concourse at SEA. How can we tell? They painted parking spots for it on the ground.
You can see some more views of the paint here.
And perhaps there is reason to be optimistic. A couple days ago, Boeing conducted the first self-powered taxi tests for the aircraft. You can see the video below (source: http://boeingblogs.com/randy/)
It looks beautiful on the ground. I can't wait to see it in the air.