Doublclick published a research report today indicating that half of Internet purchases occur only after a relevant search. Searches can occur sometimes weeks before the actual purchase is made. Not surprisingly, branded searches resulted in purchases much closer in time to the original search.
The skinny? If you're tracking the search term used just prior to purchase you're likely using data that's only a small part of the search-to-buy research equation, missing a great deal of generic search terms.