From the Oregonian:
Japanese officials have confirmed that a huge dock that washed ashore on the Oregon coast is debris from last year’s tsunami – the first to reach the state from the disaster in Japan.
The tsunami cut the dock loose in Misawa, a northern Japanese city struck by powerful waves on March 11, 2011, said Hirofumi Murabayashi, deputy consul general at the Japanese Consulate in Portland. Full article
Video report from KATU Portland last night, before the origin of the dock was confirmed.
Here's a blown-up photo here.
Here's the latest projection of debris distribution from NOAA: