In one of the first episodes of season two (I think it was Redemption) Rigsby mentioned Cho was dating someone. Do we just not know who that is yet, or have I missed something?

Yeah it's the niece of the lady Cho and Rigsby met in "Red John's Footsteps" who opened the post office box for them. The lady showed her niece's picture to Cho who said she was attractive and then Rigsby got her to give him her niece's phone number so he could give it to Cho.