Unbelievable. I just saw on my local news a story describing a "social insecurity" calculator on NY Senator Charles Schumer's web site. I followed the link, already knowing what I could expect. Schumer didn't disappoint. It took me all of about 10 minutes to rip holes in their calculations. I'm no economist, though, so I'll give you some links to a few places that can explain it better than I.
Andrew Roth over at Social Security Choice
His Update, containing links to (among others):
The Heritage Foundation
The Cato Institute (includes their own calculator)
A couple of posts at Political Calculations