I don't usually propagate YouTube videos, but I think this one makes a solid point:
In all seriousness, I do think there is room to be concerned. In the first bubble, the big paydays were the IPO. Of course many who did IPO had only a brief success while many others never got there. There were other exits, but the IPO market was a huge propeller.
This time around it seems like there are almost no IPOs, and the major exits are the tech giants. Google, Microsoft, Yahoo, EBay, etc. are the main sources of the 'crazy' valuations. What is going to happen when these guys slow down their acquisition pace?
Either way, enjoy the video.