HFCS is the abbreviation for High-Fructose Corn Syrup. And maaaan, is that stuff everywhere. It's in soda, candy, cakes, juice, coffee drinks; you get the idea.
I avoid it because I prefer sugar. I'm an arrogant, snooty gourmet and as such demand only the very best ingredients in my swill. And much like hydrogenated oils, I have seen increasing evidence to support my position vis-a-vis health.
This most recent study comes out of Princeton, where they have smart people (or so I've heard). Seems simple enough. They fed one group of rats HFCS and another sugar. The group fed HFCS got really fat, the sugar group didn't.
They pointed out that even rats fed a high-fat diet didn't gain weight to this degree. No other experimental diet except for HFCS has shown these results. That's pretty stark.
Remember, though, that what they're testing is High Fructose corn syrup, not plain ol' corn syrup. HFCS has been chemically altered to bump up the fructose levels and, possibly importantly, free the fructose from any chemical bonds. You can still safely use corn syrup in your baking.
But HFCS is appearing to be different. I would say that either reducing your HFCS intake or eliminating it completely would be a good course to take. Sadly, this would mean avoiding most sodas. If you still want a soda now and then, I seriously doubt that it will hurt you, but it should prompt you to treat a can of soda as much more of a treat than most people have been up to this point.
A sweet problem: Princeton researchers find that high-fructose corn syrup prompts considerably more weight gain (Princeton.edu)