Cannondale Bad Boy SS conversion
I'm guessing that they were talking about replacing the rear cassette with a freewheel. However, you can convert pretty much any bike with a rear cassette to a single speed with single speed conversion kit like this one: 

http://www.universalcycles.com/shopping/product_details.php?id=1764&amp;category=2621

And a chain tensioner like this one:

http://www.universalcycles.com/shopping/product_details.php?id=47140

NB: I don't necessarily recommend either one of those products or the store that I found them on. They're just examples. You can actually even "hack" your own single speed conversion by dismantling a couple of old cassettes and using the spacers in the same way that you use the larger spacers on the ready-made conversion kit.