A lot of business that would possibly have gone to pawn shops is on eBay instead - fortunately. There's obviously a big market in the sale of used stuff.
As Andrew points out, pawn shops have been around for ages. Check Cashing shops have been around in big cities for ages as well, though in the past they often weren't dedicated shops (e.g. large department stores would cash checks for customers (at least Rich's did in the 1960s)). I do think they're more visible in smaller cities than they were 30 years ago, but that's just my impression.
I think Yunus has a valid point, but I don't think it illustrates a slide of the US economy as a whole. There's certainly a market for things like [link|http://www.grameenamerica.com/AboutUs.aspx|Grameen America]. I think [link|http://www.businessweek.com/magazine/content/05_06/b3919046_mz011.htm|WalMart] recognizes that as well...
Too many traditional banks have poorly served poor and lower-middle-class areas in the US. But there seems to be a structural issue, or a lack of imagination, as well. Otherwise, one would think that smaller organizations like credit unions would serve that market far more than it does.