Ah, you're right.  For instance, Warhol's famous Cambpell's cans. 

Where I'd be concerned is if you are making props for sale.  Let's say you have created a box of Hamburger Helper for your new fabulous Poser Kitchen set, and you clearly show the trademarked name Hamburger Helper and the brand -- is it Betty Crocker? -- on the texture for the lovingly modled vertex box.  If you're making art from your own prop, yes, I think you're fine.  If, however, you sell the Fabulous Poser Kitchen set with the texture of the box art, then I think you could be in trouble.  In that case, you'd be profiting from the sale of their art, not your own artistic impression and interpretation of their art.

But, of course, if you made your prop for "Beef Boy," instead of some trademarked name, then, you're fine.  (And personally, I find making up products to be loads of fun.) 

And there's a difference between copyright and trademark.  In the hamburger helper example, the Betty Crocker spoon is a trademark, the little hand guy with the dorky expression is a trademark, and the name Hamburger Helper in the distinctive font and coloration is also a trademark.  But the photo of the steaming heap of "cheezy" gunk on the box is copyrighted.  It gets confusing.