Google started showing a few ads in the Play Store phone client earlier this year, but the feature has been in testing this whole time. Play Store ads are now available to all interested parties, so get ready to see a lot more of them.
Nothing is changing with respect to how the ads are displayed. They'll appear at the top of the results list with a small yellow badge marking them as ads. The option of placing ads in the Play Store client will start rolling out today to all advertisers and devs who use the search app install campaign for AdWords.
It's not clear if you'll be seeing ads in every search or if Google's going to ease us into things. Either way, you'll probably get used to them. The ads will be promoting Google Play content, so at least they'll blend in.
- AdWords Blog