Web 2.0 is causing a splash as it stretches the boundaries of what Web sites can do. But in the rush to add features, security has become an afterthought, experts say. The buzz around the new technology echoes the '90s Internet boom--complete with pricey conferences, plenty of start-ups, and innovative companies like MySpace.com and Writely being snapped up for big bucks. And the sense of deja vu