Yesterday I described how to record the IQ data from the USRP focusing on generating unique file names every time the GNU Radio / GRC script is executed. Playing the recorded IQ data is really simple: We take the corresponding receiver application and replace the USRP source with a file source and a throttle block.
The recorded IQ data is a simple binary file containing nothing more than the received samples. There is no information about frequency or sample rate which is why we need a throttle block to maintain a constant sample rate in the receiver.
My experimental application was for reception of APT signals from the NOAA weather satellites so my playback application is a WFM receiver. You can get the example .grc file here.
And here is a video showing the playback in action. The receiver can be used exactly the same way as when it receives the samples from the USRP. A really great and very useful feature in GNU Radio!