Google is now reaching back into your previous search history and presumably choosing a better previous search if the current one is not sufficiently monetizable.
- Is the goal of Google to increase the fill-rate of ads or to show more valuable ads in general?
- What criteria is used to reach back into a user's history? Boolean commercial/non-commercial then select last commercial search versus choosing based upon some selection algorithm from the last N searches (see previous point).
- Will the reach-back cross a topic boundary or is it only to enhance context for an ambiguous search?
- What effect will this have on the Google Keyword Tool that helps advertisers forecast demand and price for a keyword? The volume numbers must now be adjusted by the amount of time the impressions are shifted to alternate keywords.
- How much will this starve the long-tail of searches? Depending on the aggressiveness of the selection then long-tail searches may suffer a decrease in volume for adwords.
It will be interesting to watch what happens. This is likely not a non-trivial change in the keyword market.