I think this is quite a risky wager. I'm not sure how much of external factors are accounted for. The economists and financial journalists are quite excited about the whole QE and ZIRP effect. It's been deemed an unprecedented experiment. Cheap money has flooded the market. If we look at each industry of itself, most of them are in the similar state. Housing has become very expensive, stocks are at all time high, executive compensation has increased, commercial real-estate is popping up everywhere and the wealth gap is broadening. So when someone looks at statup evaluations and see them getting higher and higher, an expectation of market correction is in order just as it would be with the stock market. Perhaps the inevitable will get delayed once Euro takes up on the QE.

P.S: I think when someone talks about bubble, it doesn't imply that the current crop of startup ideas are bad. It just reflects inflation in the VC market.