Two days ago we cited a glowing report from the Interactive Advertising Bureau on ad spend for the first half of 2008. Now the gloom and doom is starting to sink in. Several analysts are lowering their estimates for online ad revenues ahead of the Q3 earning report period, according to PaidContent.

The good news (sort of): offline looks worse than online and indeed some Web-based companies could benefit as more firms look to cheaper and more targeted online ads.

First up is USB Internet analyst Ben Schachter who, while not giving specific numbers, says that while the first two months of Q3 “were decent,” September has proved difficult. While all companies are being negatively impacted to some extent, Schachter has faith in Google because of its dependence on search which remains more attractive to marketers than display.

Over at Wachovia, media analyst John Janedis sees total U.S.. ad spend slipping 0.8 percent this year and next year. Janedis had previously called for growth of 1.2 percent in 2008 and 1.5 percent in 2009. Online has been revised downward as well: 2009 spending will only grow 10 percent rather than 15  percent according to Wachovia.

It’s not much better at Deutsche Bank. Analyst Doug Mitchelson, who covers entertainment companies, slashed his ad revenue forecast for the sector by $502 million for 2008 (down to $44.1 billion) and by $2.36 billion for 2009 ($42.37 billion). Mitchelson points out that entertainment derived 29 percent of its revenue from ads in 2008. Nevertheless, Mitchelson says that entertainment companies in general should be able to manage the global credit crisis better than other firms.

The full report including charts here.