If you had paid too much attention to the risks associated with owning Internet stocks over the past year or two (like the ones this column kept harping on), you would have paid a high price in the form of missed opportunity. However, at

these

levels (words I'll live to regret, no doubt) it might be wise, just for the heck of it, to think about what could go wrong. (Remember Dell

(DELL) - Get Report

?)

It's one thing to compare some of these companies to

Cisco

(CSCO) - Get Report

, which in its earlier days -- when its stock was a rocket -- traded at 10 times forward revenue. It's another to blindly believe in, say, an

eBay

(EBAY) - Get Report

at 20 times projected gross annual sales of $1 billion -- and we're being conservative by using gross sales, not actual revenues. If you use actual revs, and generously figure that the company will generate $100 million this year, you're still dealing with a stock that trades at a hefty 200 times forward revs.

The purpose of today's sermon isn't to try to pop bubbles; it's to remind you that every company, especially those that are growing quickly, has risk.

For example:

eBay

: There's no debate that it has rapidly developed a strong brand and loyal following. But it also charges a hefty commission, while an alliance between

Yahoo!

(YHOO)

and

Onsale

(ONSL)

does the same thing for free, relying instead on ad revenues. What if eBay is forced to revise its business strategy and switch to advertising? (Of course, you could argue: What if eBay

adds

advertising?) There's also a flood of competition both on- and offline and a risk -- just a risk -- that someone will aggregate local auction sites. How would that affect eBay? (It may never happen, but you've got to consider the possibility.)

Yahoo!

: Much of its revenue growth has come from sponsorship deals, but the pace of those deals is believed to have slowed and the prices of those deals are believed to have plummeted.

Buy.com

, according to my spies, reportedly did a deal for less than $1 million. (Buy.com wouldn't comment.) What if the slowdown continues, or if these deals are done at even paltrier prices?

America Online

(AOL)

: See Yahoo! above. And what happens if subscribers, realizing they can get everything AOL has on the Internet, switch to

EarthLink

(ELNK)

,

@Home

(ATHM) - Get Report

or some other provider? And what would happen if international growth were to lag expectations?

Amazon.com

(AMZN) - Get Report

: What would happen if a company like, say, Buy.com starts aggressively undercutting Amazon, forcing its margins to fall by half? And Amazon recently raised $1.2 billion in a convertible debt offering. What would happen if it got in a cash squeeze and couldn't make the payments on that loan, or repay that loan? (Just asking.) Eventually it will have to. And Amazon recently bought a stake in

drugstore.com

, presumably using borrowed money (see the convertible debt). What happens if drugstore.com doesn't deliver the payback Amazon expects? (Fabulous site, by the way; have you seen it? Unfortunately, there are other good drugstore sites, too.)

CNet

(CNET) - Get Report

: Aiming to raise $173 million in a convert offering (see Amazon, above, for risks); runs a well-respected online tech news publication, but has tough competition on the news-gathering front from

ZDNet

. A risk, no?

Preview Travel

(PTVL)

: Great for making travel reservations, but one of the pickiest of my jet-setting colleagues prefers

TheTrip.com

in what has become a crowded field. (What if others do too?)

DoubleClick

(DCLK)

: Smart folks; good service. They count the ad spending of other companies as

their

revenue, for placing those ads on other Internet sites, while the typical ad agency just counts commissions of around 17% as revs for the same service. (Okay, DoubleClick tracks who uses the ads, but what if Wall Street finally figures it out and treats it like just another ad agency?!) And what if

AltaVista

, a very large customer, ever decides to do biz elsewhere?

EarthLink and MindSpring

(MSPG)

: Both very good services, but what if users migrate to cable or ADSL (if and when it ever becomes available)?

@Home

: Everybody who uses it loves it -- when it works the way it's supposed to. And what happens if the cable companies can't get their acts together and roll out the service any faster? (I'm still waiting, and it's been a year since my cable company said it would be a year.) What if ADSL or some other broadband technology leapfrogs cable? What if, in the end, cable gets too many users and feels like the precable days? (Just a what if.)

Lycos

(LCOS)

: What if it can't ever overcome being a third-rate service that makes third-rate merger decisions?

Infoseek

(SEEK)

: Now that

Disney

(DIS) - Get Report

owns a big stake in the search service, it's stopped taking porno ads. What if in the process it lost a big chunk of revenue? (Infoseek won't comment on that, other than to say it weighed all the implications of its move.)

The list could go on, but you get the picture: These are risks that may never materialize. (DID YOU HEAR WHAT I SAID? THEY MAY NEVER MATERIALIZE, SO GO BUG SOMEONE ELSE BEFORE YOU SEND THAT EMAIL!) But just in case they do, this column, as a public service, didn't want you to feel you weren't warned.

Herb Greenberg writes daily for TheStreet.com. In keeping with the editorial policy of TSC, he does not own or short individual stocks. He also does not invest in hedge funds or any other private investment partnerships. He welcomes your feedback at herb@thestreet.com. Greenberg writes a monthly column for Fortune and provides daily commentary for CNBC.