I'm not so sure about the cold, hard cash thing. I've always been a cash buyer, and 20 years ago it definitely made a difference. Now, the dealers are happy to get the financing business because it means a steadier revenue stream for them. The last three cars I've bought (since 2001), being cash in hand hasn't generated any bargains.
Being prepared to walk away, now that's a way to get a deal. When I bought my most recent car, I called the salesman of the other car to make the short list to tell him "thanks, but no thanks." He put me on to his manager, who was ready to drop some big numbers. Too little too late, though.