I think this fight would be fascinating. Everyone expected Henderson to knock Shields silly and for a few moments it happened but Shields showed toughness nobody expected and eventually out-wrestled and beat down Henderson. It was a very GSP-esque performance. It would be really interesting to see the game plan each of these guys would use. Obviously both guys favour top control (though Shields does better when he has a guy's back) but usually when you have two wrestlers or two high level black belts fighting it winds up being a striking match. I think everyone wants to see that from GSP again, too.
Also, it should be mentioned that Shields & training partner Nick Diaz have also been calling GSP out. Hopefully all the pub the UFC is giving Shields lately isn't a tactic to get him to come over for less money (he looks like a wimp if he doesn't).
I think it would be a lot like GSP v Hughes II and III, where GSP resorts to standing and striking with a great wrestler with suspect stand-up. It would probably go the same way as those fights too, with Shields failing to complete all/most of his takedowns, GSP picking him apart from range and only looking to take it to the ground to score points.
+1 Something like GSP/Fitch, maybe? GSP jumping on him when he is hurt but otherwise just picking at him and being patient.
I'd like to see this fight too. If Shields signs with the UFC I hope they give him an instant title shot. Having him fight Fitch or Alves beforehand might ruin the heat he has on him right now.