Even though Google failed miserably and shut down the old Google phone store, it looks like they're giving it one more go. Right now, Google is selling an unlocked Galaxy Nexus that works on both AT&T and T-Mobile for $400 on Google Play.
It's being sold in the new 'Devices' section of Google Play. You can totally imagine the store becoming more robust if the rumors of the Nexus tablet are true. It'd be interesting to see if Google will promote other devices on the Play store as well.
Other than not being locked to a contract and having an unlocked phone, the big benefit of buying the unlocked version of the Galaxy Nexus is you get the unadulterated Android experience, what Google wants Android to be, pretty much. Google specifically mentions that the Galaxy Nexus on Google Play comes with Google Wallet (and a free $10 credit). [Google Play via Google Mobile Blog]