I think the difference is public vs private money. If we are in a bubble (which I'm not sure of but let's assume yes for the sake of argument) the investors that stand to loose their capital are all "qualified investors." There are few public companies that are "frothy" this time. Personally, I think the fact that the public markets appear to still be functioning well is _why_ few startups are going for IPOs.

Of course, if startups do start to IPO and the public markets catch the over-valuation bug, then we could see a repeat of 2000. Barring that, I think a correction would not infect the markets at large and may drive more capital into the public companies that are not over valued.