The bubble of the 90's had a built in detonator in that the discipline of the public markets require a company to stand up on its own two feet. So all those companies whose liquidity event was a public offering found themselves pretty much high and dry when the balance sheet didn't meet the prospectus hype. Of course by that time, the investment bankers had long ago got paid, the early non-lock-up investors had gotten out and the mom and pop investors (and perhaps a few greedy day-traders) were left holding the worthless paper - or so the PR spin went. In other words, with all apologies to Ross Perot, that giant popping sound you heard was the bubble bursting.
Anyway, fast forward to today and there is no public market discipline against whom a start-up is measured. Add to that a bunch of VC firms flush with a renewable stream of cash, new liquidators (i.e. large public companies active in m&a) with coffers filled with cash and supported by very profitable businesses and you begin to wonder - if this is a bubble, so what? What would make it burst?
Of course there are obvious answers: terrorism, natural disaster, overall economic downturn (although the recent housing market woes have had little effect), etc. But anything less than that? How about a bad quarter for Google? Something that perhaps shows a chink in the armor of the ad supported online model? The shuttering of a high profile Web 2.0 start-up? I'm not sure. May this is a bubble, but it seems like it might be a bubble built to last.