Seattle's rain is a huge misconception. Occasionally it really rains, but this time of year, it's just cloudy and it barely drizzles, and lots of the time it merely looks like it's going to rain. Seattle gets less rainfall annually than Houston, Atlanta, and NYC.
I say skip the rain cover. I never use one, and I shoot with a 350D. Just don't let it sit in the rain forever (if in fact it is raining--this week we have had gorgeous sunshiny days) and wipe it off quickly after use. As for equipment, that would be a tough call, but I would bring the 17-55 and 85mm if you are just trying to keep it light.
By the way, umbrella = tourist, so if you want to blend in, wear shorts and a light rain jacket. 
Seattle is a great city and I'm sure you'll enjoy it. There's plenty to see and do, and the variety is incredible here. Be sure to hit up Pike's Place Market. It's a photographer's playground.