I don't spend too much time playing with local search on any site, but Scott Buresh, an SEO person does. He offered this observation in an article at Promotion World.

It should be noted that you will not see local search results for all queries that contain a local modifier. If you type in "Atlanta search engine optimization," for example, you will not see the geographic box. Google somehow "knows" when a geographic modifier really means that you only offer services in a particular area. In effect, it has figured out an algorithm that separates the businesses that are dealing with a local clientele versus those that are located in a particular geographic region but service a national, or multinational, clientele. Yeah, those guys are pretty good.

Only one problem: if you do that query, you DO get local results. So, Google isn't all that smart after all. Frankly, I didn't believe his contention, which is why I tried it out. But, I'll give the author benefit of the doubt: perhaps when he last did the query, it didn't spawn a local results map. I really wonder how Google decides what returns a local search and what does not?