Henry Blodget is very much a poster boy for the dotcom bubble. The former Wall Street analyst and now editor of Business Insider, knows bubbles very well. You could say he helped build Bubble 1.0 with his enthusiastic research reports, which supported lofty valuations of key dotcom stocks.

When the dotcom bubble turned into a dotbomb, some of the blame landed at Mr Blodget's feet. The SEC banned him from the securities industry and fined him for sending emails with different opinions to that of his published research reports.

Mr Blodget became the face for everything that was bad about Wall Street firms, and the way they won the business for lucrative IPOs by publishing highly positive research reports.

By all accounts Mr Blodget is very much reformed and chastened by his experience.

So when people ask "Is there a new tech bubble?" because valuations of Facebook, Twitter, Zynga and other tech high flyers are rising faster than their revenues, Mr Blodget is clearly well positioned to compare the present with the past.

He recently made a presentation on this topic and his conclusion was: "No. But there are other things to worry about."

Here are his key points:

- Valuations of Facebook, Zynga, Groupon (but not Twitter) are high but justified.
- A key difference with Bubble 1.0 is that "crap" companies are not rising.
- The tech sector appears to be in a cyclical boom rather than a bubble.
- A key worry is that US stock market fundamentals are "scary."

Mr Blodget estimates that US stocks are overvalued by about 40 percent. A market correction would have a chilling effect on tech company valuations.

You can see his slide deck here: