Is it OK to have a 20 Amp receptacle on a circuit with 12 AWG wiring and a 15 Amp breaker?
The answer to this question can be found in article 210 of the National Electrical Code (NEC). Section 210.3 of this article, tells us that the circuit rating shall be determined by the overcurrent device rating.

NEC 2011
210.3 Rating. Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or
setting of the overcurrent device. The rating for other than
individual branch circuits shall be 15, 20, 30, 40, and 50 amperes.
Where conductors of higher ampacity are used for any reason, the
ampere rating or setting of the specified overcurrent device shall
determine the circuit rating.

So if you have a 15A breaker, the circuit rating is 15 Amps. Section 210.21(B)(3), tells us to refer to table 210.21(B)(3) to determine the receptacle rating for circuits supplying two or more receptacles.

210.21(B)(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle
ratings shall conform to the values listed in Table 210.21(B)(3), or
where larger than 50 amperes, the receptacle rating shall not be less
than the branch-circuit rating.


This tells us that if we have a 15A circuit breaker protecting a circuit with two or more receptacles, each receptacle on the circuit must be rated not over 15 amperes. It's also good to remember, that the NEC does not view a duplex receptacle as a single receptacle according to the definition of a receptacle.

Receptacle. A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle
is a single contact device with no other contact device on the same
yoke. A multiple receptacle is two or more contact devices on the same
yoke.

So if you're installing even a single duplex receptacle on the circuit, you'll need to follow 210.21(B)(3) and use a 15A receptacle.