First, a disclaimer: my singlespeed weighs, like, probably 23 pounds or something. I haven't weighed it, and all I really care about is that it works.
But if you're into geeked-out lightweight race bikes, Nick's new steed might do the trick. It's a custom 29er frame and Actiontec 65mm travel fork with American Classic/XTR/Magura type parts. Pretty tricked out. I was *astounded*, though, when the weight came out at 21 pounds even.
That's really light, at least in my book. Though clearly I'm not a candidate for the weightweenie hall of fame at this point. Thanks to Nick for the geeky pix.