MMA Mania and Around the Octagon are both reporting that the Shields-Riggs fight is set, and it will be added to a card that includes Frank Shamrock vs. Nick Diaz and Scott Smith vs. Benji Radach.
Shields is universally considered one of the 10 best welterweights in the world, but because he was under contract to EliteXC and has now had that contract acquired by Strikeforce, he's unable to fight any of the other top welterweights, all of whom are under contract to the UFC.
Sow how much better is Shields than Riggs? A lot better. In my own list of the top 10 welterweights in MMA, I have Shields at No. 5, and I didn't even think of listing Riggs in that 10. (The other nine spots in the Top 10 all belong to UFC fighters.) Riggs isn't in anyone's top 10; he doesn't even register in the Bloody Elbow meta-rankings and he comes in at No. 32 in the FightMatrix welterweight rankings. This is a mismatch.