I would tend to think that part of the reason the iPhone4 .ipsw for 4.1 is almost 600 megs and the 3GS is under 400 megs is that these software features that differentiate the two models don't exist at all on the older models. I do not think it's something you can simply enable, because the software for HDR is just not on the 3GS.
That being said, and with that redmondpie.com article, i could be way wrong (I hope I am!)