Actually, you've got it almost backwards. If people are worrying that there's a bubble, there probably isn't. Bubbles form when everybody thinks things are worth more than they are. This produces a positive feedback loop whereby investors buy things they think are valuable, raising their price and therefore perceived value, causing other investors to buy.
If everybody's worried that there might be a bubble, then investors will be more cautious about buying simply because others are buying.
The biggest difference between the tech bubble and what's happening today is that the investors are all sophisticated--these aren't retirees dumping their savings into Pets.com stock.
No, I don't have it backwards. :) There's always two groups of people. The wild-eyed optimists, and the doubting Thomases. There are still tons of wild-eyed optimists right now, but for me, the emergence of a vocal minority of doubting thomases is a clearer sign of a bubble, or at least the late stages of a boom (is there a technical difference?), because it means those closest to the issue are starting to tilt more pessimistic than the investor community at large.
There are few to no "wild-eyed optimists" right now, save for a handful of investors pumping money into Zynga and Facebook. If you think that compares to 1998, you really ought to go back and re-read history.
Why is it significant? Those are two companies. Two does not compare to the hundreds of public companies that had millions pumped into them and the widespread investment in them.
If everybody's worried that there might be a bubble, then investors will be more cautious about buying simply because others are buying.
The biggest difference between the tech bubble and what's happening today is that the investors are all sophisticated--these aren't retirees dumping their savings into Pets.com stock.