How do apps/games become featured in the Android Market?
As far as I know, they're simply picked by Google's staff. I'd assume they look for the general indicators of a quality app (good ratings, lots of active installs, etc) but popular or otherwise well-known developers probably have a leg up regardless. Google has never really come out and said "follow these guidelines", possibly to reduce people trying to game the system, similar to how they don't release their full page rank algorithm.
However, I can definitely say that it is not possible to purchase your way into the featured list. Google's Android Market developer help page notes specifically:
Technically speaking, the only restriction I've seen is that your app must have a properly sized "featured" image to be used on the "Featured Apps" area. Aside from that, Google has been very mum about the process.