"Most analysts are severely underestimating the impact the mortgage collapse could have on online advertising spending," wrote Henry Blodget, co-founder and CEO of the Silicon Alley Insider blog. Blodget and other tech insiders point out that the rising interest rates and millions of dollars of debt defaults don't just hurt mortgage companies--they threaten the entire financial services sector, including investment, credit, and other non-mortgage companies.

According to Nielsen, just over a third of all U.S. online advertising dollars spent in July came from the financial sector--with mortgage and credit reporting firms representing five of the top ten advertisers. Together, those companies spent nearly $200 million on search, display and other Web advertising, meaning that a slowdown would degrade a fairly significant annual revenue stream.

But ad industry vets argue that other verticals--namely automotive and consumer electronics manufacturers--as well as the 2008 Olympics and Presidential election could pick up some of the slack.

"There may be a temporary void, but we still have the macro trend of key industries like autos and electronics continuing to shift more of their media budgets online," said Jerry von Gerichten, vice president, media director of The Halo Group. "We're going into a strong year for advertising--from spending in Q4 to January, then the Super Bowl and the primaries, to the Olympics next August and down to the campaign season blitz. I think the dollars will just continue to roll."

Market research analysts like Rick Sizemore of Multimedia Intelligence peg giants like Google and Yahoo to be hit hardest. "Google has particularly strong exposure, due to their commanding market share in Internet advertising," wrote Sizemore in a report. But while the search giant may feel "the most pain from cutbacks in ad spending from real-estate related companies, they will not be alone. Yahoo's U.S. search market share is over 20%, and MSN/Microsoft and AOL are likely to feel the pinch as well."

Others say that ad networks may face significant losses--as sub-prime lenders often purchase remnant inventory, not premium placement directly from publishers. "If these low-end mortgage firms were deploying their buys through the networks like Tacoda and Blue Lithium, the ad networks will be more affected in the short term," said von Gerichten. For some digital marketing agencies, the question is not about the loss of dollars, but how a downturn would affect overall strategies. "So far we have not seen the mortgage meltdown affecting our online marketing strategies for our real estate clients. In fact, clients seem to be forging ahead with business as usual," said Raman Kia, director of operations and integration, Morpheus Media.

Kia acknowledged that it might be too soon to gauge the direct affect of the mortgage meltdown--but that it also could force some clients to increase spending, as the pressures to unload inventory in a weak market grow stronger. Some real estate companies may also be waiting for the 0.25% reduction in the base U.S. interest rate slated for September 15.

"However, should the interest rates rise in the long term as a result of the sub-prime crisis, then we could realistically expect some damage to the luxury real estate market," said Kia. "And at that point we should expect some impact on the online marketing strategy that would ordinarily be focused on reaching high-end consumers in the market for luxury real estate."

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