Several visitors to Cyclelicious come here searching for info about the lightest bike and land on this page about a very lightweight eight pound Litespeed Ghisallo.
James has been showcasing very lightweight bikes at his blog lately. Last fall, he mentioned the 6.5 pound Crumpton fixed gear bike, and noting that this bike will be at the North American Handbuilt Bicycle Show coming up this weekend in San Jose, CA.
He then wrote about lightweight folders, with Bike Friday Walter chiming in that Bike Friday built a 15-pound folding road bike for a customer.
This discussion leaves James (and others) wondering, though: how light is too light? Read the comments at Bicycle Design for some good discussion.