I heard today that Seattle's Urban Surf is holding a weekly sup race on Lake Union every Thursday afternoon. Contact the shop for more info, http://www.urbansurf.com/.
Also in Seattle, Cheka Looka will be doing a sup race on 10:00AM Sunday, August 16th, call them for more info, 206.726.7878.
Tacoma's Backpackers Supply had a sup social for folks to demo boards and learn about the new sport. It was July 22, but keep them on the radar for future such events. Contact them at 253.472.4402 for more info.
Also check out Ken Campbell's incredible blog about sup'ing around the region and other cool stuff. Ken is a local kayak and sup guide and instructor at Backpacker's Supply: http://lastwilderness.blogspot.com/