I get similar results using other citynames... map showing with search using ZIP. The cityname is necessary, though... searching for "apartments" and the ZIP with the cityname doesn't generate the map. In one local city search I tried, there were three results...
1) BusinessName Apartments, with its own website...
2) FancyName Apartments, apparently a now-vanished listing on a nearby city rentals listing page, linked in the serp...
...and... particularly interesting...
3) one was a classified ad with the title Cityname Apartment, with the only contact info being a hotmail address. I'm thinking the ad was perhaps a Google Base result, as there's no url of any kind associated with it.
As to why only the query with the ZIP returns the map, apparently Google is looking to provide localized rental classifieds, and ZIP granularity is probably what they consider useful.
They're stretching hard to fill even three slots, but it looks like they are trying this out. I don't think Craig's list needs to worry just yet. ;)