This is primarily impacting the big brand advertisers; minimal effect on all the little advertisers. That being said, we should be seeing an economic downturn in the next year or two, which could weaken the market. Its to hard to really know what stage of the business cycle we are in because of the sheer quantity of variables at play here from housing to war spending.
On the positive side, internet advertising generally has a much higher ROI than other mediums. This could only hasten the move from old media as more companies demand accountability from their marketing departments.
My take on it is that if you have high profit margins you have zero to worry about. For publicly traded companies, including Google, a slowdown in growth (which is really what this is, not an actual decrease in spending) can have a compounded effect on the stock price because of how heavy it is tied to speculation.