????For all the talk this year about a tech bubble, there is surprisingly little agreement. On the one hand, valuations of hot web companies are irrationally high; on the other big tech valuations are inexplicably low. On the one hand, the number of overpriced IPOs in 2011 is nowhere near that of 1999; but on the other, most of the speculation is happening on private markets. On the one hand, stocks of recent debuts like LinkedIn (LNKD) and RenRen (RENN) are down from their initial highs; but on the other they are still expensive by any sensible measure.

????Even with recent IPOs down from the high points of their oh-so-brief histories, It's safe to say there is a small-scale tech bubble in the making -- relatively contained, but unsettling nonetheless. The better question is how big it could get. Will it metastasize like the dot-com frenzy of 1999? Will there be little demand beyond marquee names like Groupon, Zynga or Twitter? Will an imminent Facebook IPO pour fuel onto the bonfire, or simply mark the culmination of what will prove a modest brushfire?

????What's often overlooked in this debate is that the answers to these questions -- and therefore the fate of the new tech bubble -- lies outside the tech industry. Many web startups thrived even through the dark days of the Great Recession, and today the coverage of topics like cloud computing, the mobile web and social networks is so insular it often feels self-absorbed. This is the other tech bubble, one that few people are talking about: All this commentary and discussion is happening as if the world outside the web industry didn't matter so much, as if web startups operated inside their safe little bubble of reality.

????That attitude might have worked better a few years ago, when financing a web startup meant raising a few million from VCs to support a small crew of highly talented engineers. It's not so true now that many companies are much bigger, increasingly turning to public markets to tap hundreds of millions of dollars. And it overlooks the biggest difference between the dot-com bubble and today's web bubble: In 2011, the financial world is a hell of a lot scarier than it was in 1999.

????In the late 90s, a market-rattling crisis meant something like Long-Term Capital Management, a hedge fund that cost the Federal Reserve $3.6 billion to bail out. This was an era when people seriously fretted about the Millennium Bug, a phantom crisis that prompted the Fed to make money cheaply available so companies could upgrade their corporate software. Instead, of course, people took all that free money and invested it all too freely in tech stocks.

????Today, we look back on those 90s-era crises almost wistfully -- in retrospect, they feel like painful high-school memories: a bad report card or a rejected prom invitation. The tribulations of our adult lives feel much graver now. The Fed has again been busy printing money through two rounds of quantitative easing -- in essence, injecting money into a morbid U.S. economy -- only this time it's not to help companies fix clunky software but to try and get lenders to fix an economy that nearly froze up after the housing bubble popped. But one thing hasn't changed: All that cheap money is sloshing its way into unintended investments -- including red-hot tech startups. And that money has been driving up their valuations.

????Now the Fed has to decide whether it's going to extend quantitative easing and inject even more cash into markets. The answer could come as early as this week. The last round, called QE2, had at best mixed results in creating new jobs in the U.S., but during the past year it made it fun and profitable to invest in U.S. stocks. There is another debate happening over whether QE2 should be followed by QE3. Many voices are arguing it shouldn't. But without QE3, a good deal of liquidity that has been flowing into web startups could dry up very quickly. In other words, much less air for the tech bubble.

????QE3 might come as welcome news to someone who bought into Facebook on Sharespost and wants a lucrative profit from its IPO. But it's bad news elsewhere. For one, all that excess liquidity is helping to drive up commodity prices. Which means consumers are spending more money on gas and food. Which in turn means less disposable income for an iPhone 5 and so many apps that make it useful. More immediately, QE3 would mean something so awful has happened in the global economy -- say, Greece defaulting on its debts -- that the Fed felt it had no choice but to inject more cash into the markets.

????It's looking more and more like Greece is going to default -- the markets have recently been calculating that there's a 78% chance of this happening. A CFO in SoMa might ask, what does this have to do with the price of tea in China? Or more to the point: What does the price of Greek bonds have to do with the IPO of a web startup?

????Here's my short answer: A lot more than it did in the 1990s. Today, money managers are still skittish from the crash of 2008. It's not just Greece weighing on their minds, but that the contagion could spread to other European nations with heavy debt -- even to the U.S., if Congress bungles things badly enough. In such a volatile world, capital flows can dry up as fast as the morning dew. And in an economy where risk looms larger than opportunity, will anyone really be interested in a company, like Facebook, valued at 50 times its revenue?

????Few people in SoMa's converted warehouses seem to be talking about any of this. They are talking instead about financial bubbles: How the Groupon IPO will fare, when Facebook will file, how much Zynga will try to raise. Nobody seems terribly concerned about potential crises in global markets that could wipe away demand for any IPO -- tech or otherwise. They are toiling inside the sunny, insular bubble that the web industry has grown up inside. And in the end, that may prove to be the more dangerous bubble of all.