From my experience, and my friends' experience, in the Boston area, where they have posted real price declines, you still have sellers that are unrealistic and will pull their houses off the market before they even consider a 20% (or 10% or 5%) "low-ball" offer, no matter how much information you can give them for decent comps in the area. These sellers are a waste of time.
From what I have seen and heard, it all depends on the seller. Finally, after about two years of price declines, it seems there are more and more sellers that have equity in their homes that are really pricing them to sell, and they typically sell within 5%, +/-, of the asking price.
Seattle is at the top of the "precipice", and I would think that it is going to take a while before sellers realize that they aren't going to get the prices they necessarily think they should get. I think by Aug - Oct 2008, you'll start seeing some realization from Seattle sellers, with more sellers willing to negotiate.
Buying a house is a business transaction. Selling a house is more than a business transaction because of all the personal memories and time spent there (from my personal experience). If the sellers won't budge on their price, and you cannot afford it, then move on to the next property. If you can afford it, it is what you need/want in a property, you really want to own a home, and think you'll be there for 10+ years, then consider buying it at the asking price.
Also, be realistic. I don't expect to buy a (real) $1M property for $600K.
Personally, this is what we are going to do when we are buying our next house:
Find out as much info about the sellers that we can: when did they buy, how much of a down payment did they put down, how much of a mortgage do they have, how long has it been on the market, how many price reductions, how much $ and sweat equity did they put into it, why do they want to move, where are they moving, etc. That will give me an idea how willing the sellers are to negotiate their asking price.
Then, once I think that the sellers could be willing to negotiate, look up all the recently sold comps (3 months is best) in the neighborhood AND current listings (their competition). It may be that the sellers have priced their property pretty well and we'll likely give an offer 5 - 10% less than the asking price. If the sellers have really overpriced it, then we'd do 10 - 20% less (or whatever is appropriate) and give them our supporting comps/listings.
Just my 200 cents...