Alvin is correct, he fought for the UFC in the SEG days. Frank called Dana White an Uncle Fester look alike. He said the only way he would go back to the UFC was for $50 mil and half the company. He said this in an interview I read a little before his comeback fight with Cesar Gracie.
But as Alvin said, with Tito and BJ coming back after leaving on such bad terms, anything can happen. As long as Strikeforce is around I don't see Frank going to the UFC. He gets to call the shots in Strikeforce pretty much.