I am not a financial specialist and this isn't financial advice, but just judging from the business cycle, my guess would be that we're due for a downturn around 2016. If things get particularly rough the gravy train could stop this year.
But just the fact that there's talk of a bubble tends to be a "contrarian indicator".
P/E on all the flagship tech stocks is just totally insane. My guess is that they're in for a Time-Warner, Meet AOL moment. Twitter looks like a great candidate for the Pets.com of the 2010s.
But remember: markets can stay irrational longer than players can stay solvent.
That said, part of the reason I suspect that the 'startup' model might 'survive' is that to investors, funding dollars are pocket change. Likewise the startup model essentially allows a capital firm to externalize risk, management, R&D, etc. while maintain a minimal but guiding presence via the board and whatever bizarre equity structure they dreamed up. Even more to the point, most companies are holding historically large cash reserves, which is one source, I suspect, of the dollars getting thrown at startups. I'd expect that to dry up when the economy contracts, or when cash starts flowing into more traditional investments.