Based on county projections from July 2006 estimates, here's how a 10th district might be placed in WA. District 1 moves up into Snohomish county and takes up most of the county. District 2 stays west of the cascades, but now links Bellingham to Bremerton across the islands. District 3 has to move west along the Columbia and would stretch from Vancouver up to Kennewick. District 4 links Yakima and Walla Walla, which district 5 gives up to keep to the new smaller sized districts. District 6 remains in Tacoma but only extends to the near suburbs like Lakewood and Puyallup. District 7 and 8 are much the same being Seattle and Bellevue/eastern King/eastern Pierce respectively. District 9 could be entirely in King stretching from Renton to Federal Way. And then the new district 10 is formed from Olympia and the Olympic peninsula all the way down the Pacific coast.
I'll leave it to locals to determine the likely partisan makeup of those districts. 
If Washington gains a 10th district, the Republicans can thank illegal immigration. Those two trans-Cascadian districts have enormous non-citizen Latino farm worker populations that bulk up their numbers but don't directly affect the vote totals, except insofar as there's a conservative backlash to their presence.
