Google Maps' ranking factors and algorithm are being researched very thoroughly by local search expert Mike Blumenthal. Mike's got several posts about the algorithm's factors, including some noteworthy comparisons to Google's OneBox results.
For those of you who like more old-school type SEO, Mike Blumenthal has a great post concerning directories that Google cares about in regards to its Maps algorithm. The directory factor still carries weight apparently, though as in the general search algorithm, that weight has greatly decreased since its heyday.
The cutoff to have a star rating, from looking at Mike's data, seems to be roughly 10-15 reviews; any less and no stars will appear. This is likely to avoid skewing the data based on only a handful of reviews.
In any event, I encourage you all to subscribe to Mike's "Understanding Google Maps and Yahoo Local" RSS feed if you're interested in local search. His blog is a great resource both on optimizing for local search as well as for local search news.