NEW YORK (CNN/Money) - Every life has its fair share of what can be called, for the lack of a better term, Oh-God moments.

No, nothing to do with the movie -- we're talking about those times that set you to bargaining with the Almighty. "Oh, God, if you just get me upstairs without waking Mom and Dad up, I'll walk in your ways and get good grades." "Oh, God, if you just keep the SEC from looking too closely at the second-quarter books..." You know the drill.

A little less than a year ago, many investors were undoubtedly cutting deals with their maker. Stocks were careening downward, with the S&P 500 reaching its lowest level in more than five years. Anyone who held to his stocks on the way down, particularly those who held on to busted highfliers like Corning or Broadcom, was in serious pain. The sort of pain that makes you say things like: "Oh, God, if you just make Avaya go up to $4 again, I'll sell it and invest conservatively and live within my means for the rest of my days."

Well, Avaya (AV: Research, Estimates) trades at $10.46 now, and is up 607 percent from its October lows. The communications network and services outfit now trades at 44 times next year's expected earnings. It is also one of the 20 most widely held stocks in Merrill Lynch accounts.

Avaya's not the only AT&T spinoff on Merrill's top 20 list that's seen its fortunes improve markedly. There's AT&T Wireless (AWE: Research, Estimates), which is up 180 percent and trades at 41 times next year's earnings. There's Agere Systems (AGR.A: Research, Estimates), up 424 percent and trading at 62 times next year's earnings. There's Lucent (LU: Research, Estimates), which is up 254 percent and which you can't put a P/E multiple on because it isn't expect to earn anything next year.

Other big winners on the Merrill list include Intel (INTC: Research, Estimates), up 117 percent, and Cisco (CSCO: Research, Estimates), up 126 percent.

And the amazing thing is that such resplendent highfliers show no sign of letting up. Stocks like Sanmina-SCI (up 314 percent) seem to add to their gains every day. Given all that happened during the past three years, why isn't anybody taking any off the table?

"Greed's keeping everybody in the game," said Brett Gallagher, head of U.S. equities at Julius Baer Investment Management. "It's getting very, very dangerous."

This isn't to say that at least some of the big moves we've seen this year are not justified -- perhaps they are. But we are talking about stocks that are, by definition, risky. For valuations to be justified their earnings must grow gangbusters and they must maintain strong earnings growth characteristics for years to come. An investor who holds these stocks has taken on a certain amount of risk, and because they have risen so much in value relative to, say, the S&P 500 or a Treasury note, that investor's holding is more risky than it was last October, not less.

The rational thing to do when your portfolio gets risky is to adjust it. But the real world doesn't seem to work that way.

"I don't think that people have the risk equation in their mind when they invest," said Santa Clara University finance professor Meir Statman. "People who vowed to themselves that as soon as a stock got back to $5 they'd get out, now that it's at $5 they say, 'Maybe this is a $10 stock.'"

Or, thinks Steve Henningsen of Boulder, Colo.-based financial adviser The Wealth Conservancy, it might be that investors' portfolios have become so battered that they're showing the characteristics of amateur gamblers.

"You sit at the table at Vegas and you're down," Henningsen said. "What do you do? You don't spread your bets around the table; you put it all on that red 30."

Henningsen believes that too many people are playing the market like it's a game, and they don't realize exactly how much they're hurting themselves. When clients -- typically people who have inherited large amounts of money or retirees who have built up a store of wealth -- come to him, one of the first things he does is ditch individual stocks. The way he sees it, putting any money at all with a single company is like putting down your chips on that red 30. Particularly since you may already own a piece of the company through one fund or another.

"We don't believe in individual stocks because of the inherent risk in them," he said. "Our clients can't afford big blowups."