I'd say there're two kinds of startups which are unknown to mainstream America. One is the kinds the general public have actually used, but just may not realize that is a service provided by a startup company. This includes the likes of Slide or Meebo, which many more people have put their widgets on their blogs/Myspace than going on to their proprietary website. These startups are fine; they may just need more PR to the mainstream media.

The startups that are really hard to reach mainstreams are those me-too startups, which are those "social network for XX" or "youtube cross wikipedia" kind of thing that users could not tell a slight difference from the sites that they have been using.