The IPO pipeline remains vibrant, but the spigot to bring the deals to market—a reflection of investors' appetite for risk—-has been closed to a trickle in early 2008, according to market researchers.

Worldwide, 28 initial public offerings expected to raise $10.5 billion were postponed or withdrawn through January 30 versus 10 deals worth $374 million in the 2007 period, Dealogic reported.

Among the companies whose debuts were shelved were Imperium Renewables, a cleantech firm that pulled a $345 million IPO and Light Sciences Oncology, a life sciences company that had planned a deal worth as much as $84.8 million.

Reflecting the unsteady atmosphere, Renaissance Capital's IPOHome.com site reported that as of February 15, 37 IPOs were filed, 6 percent more than the 2007 period, but only 17 IPOs worth $3.9 billion were priced, 50 percent less than last year.

Maria Pinelli, Americas leader of strategic growth markets at Ernst & Young, said that, if anything, the lineup of companies in the IPO pipeline is stronger than last year. As of the end of January, Ernst & Young counted 100 companies in the IPO pipeline, 10 more than at the end of December. The average deal size for the 2008 period was $281 million versus $187 million at year's end 2007.

"What's going to cause a company to withdraw or delay is pricing," she said. "To the extent volatility in stocks hits on a day, it's going to drive down the overall pricing of the market…If we can see some stability and momentum in the market, some of these companies might come out and surprise us."

Dealogic also reported a sharp decline in 2007 for IPO listings on the London Alternative Investment Market. The market, which has sought to lure venture-backed startups, raised $12 billion in 181 deals in 2007, a 24 percent decrease from the $15.7 billion raised in 243 deals in 2006.

Companies that are forced by market turbulence or other factors to postpone their IPOs could find a slightly chillier reception the second time around.

A 2006 academic study from Qin Lian, a professor at the University of Alabama, found that second-round IPO valuations were 24-39 percent lower than those of first time IPOs.

And, in the end, whether an IPO deal gets done ultimately comes down to dollars and cents.

"It all comes down to pricing," said Ms. Pinelli.