Mike Blumenthal just did a post re:
Local Carousel Ranking = Maps Ranking = Location Prominence
"As Dave Rodecker pointed out the other day in his comments on the roll-out of the Local Carousel, the ranking algo that Google is using is the same location prominence algo used in the current/old Google Maps. There does not appear to be any blended/organic influences in the results that I have looked at so far and the ranking matches pin for pin in both Maps and the 7-Pack display for the same search results.
Traditional Location prominence factors of citations, reviews, branded links etc are more important than pure web rank."
What I'm seeing, in comparisons I've done, is that the new Carousel ranking exactly mimics the old "pack" ranking order. And the old "pack" ranking order is still primarily based on organic ranking factors not location prominence or necessarily # of reviews or citations.
Here is an example that compares: old PACK and new CAROUSEL and Proximity

So there you can see that A and B for example are further from centroid AND have fewer reviews
than E, F and G near downtown.
PF Changs, which should be highly prominent, probably has tons of citations, is highly branded, has 110 reviews (vs 30 and 10 for the restaurants in the A and B spot) AND is much closer to downtown than A and B are... is not in the pack or the 1st group of results in the carousel. Changs is on page 2 of maps and you have to scroll carousel to find it.
So to me it does not look like the carousel order is based on reviews, citations, location prominence, but the same blended algo we've had for awhile now that is still heavily weighted toward organic ranking factors. You can see this by doing the same search in AOL which pulls pure organic results from Google. BUT it's super hard to decipher for restaurants because tons of directory, Yelp, Allmenues, UrbanSpoon and other pages get in the way. Easier to see the organic influence on AOL for Dentist, Plumber, Chiro and Attorney searches.