Just after initially unveiling Google Play Services, Google has made the APK installable directly from the Play Store.
For those who missed the announcement, Google Play Services is an APK rolling out automatically to devices running Android 2.2+ that will allow Android apps to easily integrate with Google services like Google+. At present, the app (which is in version 1.0 "Asiago") includes components and relevant client APIs for OAuth 2.0 authentication, Google+ sign-in, and Google+ +1 buttons. Google assures users that "we anticipate more Google services to be continuously added, so be on the lookout for these updates."
The Google Play Services app began rolling out through the Play Store automatically, primarily in order to keep updates to the service independent of carrier or OEM updates.