According to a report from 9to5Google, an affiliate program for the Play Store is in the works that would allow individuals and companies to earn money by directing people to content in Google's ecosystem. This would start with music and movies, but could expand to cover just about everything that Google sells.
The program would be similar to the affiliate programs already operated by Amazon, eBay, and (especially) Apple. Basically, you'd have an affiliate ID that credits you for part of the sale when someone buys content via your links. 9to5Google alleges the system will be powered by Performance Horizon Group, which also backs Apple's iTunes affiliate platform.
Google's affiliate program is reportedly in the early stages of development, so it could still change dramatically or be scrapped completely. The current plan is to start with music and movies, then add in apps/games and possibly even hardware on the Google Store. Google has, predictably, declined to talk about this report.