Is the CC2531 Dongle already FCC certified?

Probably need someone from TI to verify, but FCC certification depends on your specific implementation (antenna, RF components, peripherals, etc). Although intended for specific protocols (Zigbee, SimpliciTI, etc), the CC2531 has thousands of different possible RF configurations for frequencies, bandwidths, duty cycles, output power, etc that can't possibly all be tested and guaranteed to pass (in fact, is it possible to set up the CC2531 to fail FCC). Your application will most likely need FCC testing and certification in order to be sold in a commercial market. 

I don't know the exact terms that FCC uses, but since TI is not selling a consumer end-product, but rather only a component, it is the responsibility of the end product manufacturer to receive FCC certification. At least that's my understanding...