I believe that is no longer the case, although I don't know when it was changed. I looked into this last October as I was shopping for my first "metal" 'book, and I live with very weak wi-fi reception (over the hill and through the woods, so to speak).
I read and was told that the newer metal 'books have the antenna placed in the plastic hinge area (as do the plastic 'books), so reception across all should be the same.
Since there was still a re-stocking fee at the time, I got the Apple store manager's approval for a no-fee-restocking if I found that the MBP's wi-fi reception was any worse than my two plastic 'books.
I brought home, set it up next to my iBook and right where my (RIP) 2008 Macbook had always sat. There is one place where I can *just* eke out three bars, but usually get two bars. Then on the couch I can usually *just* get two bars but sometimes only one bar.
The 2010 MBP's reception was exactly the same as the iBook and plastic Macbook, from what I could tell. I kept the MBP. (Of course now I'm reading that the 2011 MBP does even better than ever thanks to a new antenna.)
So, while I agree that older metal 'books had wi-fi reception problems (my brother had a series of them back when I was using my iBook and my reception was then better than his); I don't believe that is the case anymore.
OP: Are you using a router with the short, black antennae that you can aim? If so, you might try making a free, "Windsurfer" to put on it. This is basically a specifically-sized-and-shaped "butterfly/taco" of paper and aluminum foil that you plunk down on the antenna(e). It gained me one to two bars in my "over the hill and through the woods" setup. I used to have to use an AC-powered Buffalo antenna booster, but no more!

You can find exact plans free on the web. My router has three of those black antennae, and I put Windsurfers on two of them (the three antennae are close together so could not fit one on the middle one).
Huge, huge difference. Metal buildings included!
Miss Terri