According to the 2008 National Association of Realtors Profile of Home Buyers and Sellers - 

62% of Home Buyers reported they used the yard sign as part of their search for their new home.

That's 2,697,000* prospective home buyers!

*2008  Single Family Home Sales  - 4,350,000

And, that same survey reported -

15% of home buyers found the home they purchased as a direct result of the yard sign.

5-Times more effective than newspaper!