While "intuitive" domains (direct navigation) can generate considerable traffic, Stephen Baker at BusinessWeek asks whether Marchex's previous acquisition of ZIP code-based domains will prove a meaningful local search strategy.
The headline of the post is "Could zip codes burn Google in local search?" Short answer: no. Relatively speaking a very small percentage of users search for ZIP codes (as opposed to place names). The Marchex portfolio of ZIP sites is just part of a larger proprietary network the company is building to boost the quality of its traffic.
The ZIP sites will need to be aggregated with some sort of "front door." Otherwise, they'll just catch occasional, random consumer searches but won't make a meaningful impact on local search in general.