Groupon's rejection of Google's bid indicative of a bubble inflating?

A Silicon Bubble Shows Signs Of Reinflating - NYTimes.com:

Google was willing to pay $6 billon for Groupon, an online coupon company that was valued at $1.35 billion only eight months ago. And Groupon was willing to reject the bid on Friday evening, presumably because it could sell for even more money later.

Less than a decade after the dot-com bust taught Wall Street and Silicon Valley investors that what goes up does not keep going up forever, a growing number of entrepreneurs and a few venture capitalists are beginning to wonder if investments in tech start-ups are headed toward another big bust.