How is 'Local Business Results' listing order determined?

With Google's "Local Business Results" how is the order of the listings determined? For some time now, it has appeared that the order is determined by who is closest to the center of town, but I'm starting to see that change, and can't see what's determining who's #1, #2 etc. I've been thinking of setting up some office space in the center of town where I do most of my business in order to come up at the top of the list, so it would be very good to know if this is changing before I make such an investment.

I've created and managed several local campaigns recently and from what I can tell, getting top listings depends on 3 things. 1. Proximity to the business center (important, but not a deal breaker) 2. Ratings and reviews - very important. Google is very keen on user generated content and loves user ratings / reviews. 3. Use of keywords in the business name / title and throughout the listing.

Listings that are fully filled out seem to get better placement, but it may take some time to overcome older listings. I have been able to get into the top 5 within several weeks and top billing in several months for all my listings, so far.