"Yahoo! Search Marketing initiated a systems upgrade this past weekend to lay the groundwork for performance and availability improvements that our advertisers will see over the coming months," said a Yahoo! spokeswoman in an e-mail. "During this process, we encountered some unexpected issues that affected advertisers' ability to access their accounts."

Not all search marketers appear to have been affected to the same extent by the problem; one large search marketing firm told OnlineMediaDaily it hadn't noticed any difficulties with Yahoo!'s service this week.

At another search engine marketing firm, Enquiro, the problem only became apparent on Wednesday, said Cherise Slater, sponsored search strategist at the company. She said that as of 7 a.m. Wednesday, it was impossible to log in to manage campaigns. By early evening, Enquiro staff was able to get on the site, but was experiencing significant delays--in some cases, significant enough that users weren't able to make changes before being "timed out" of the page.

But, she said, clients' ads were appearing as sponsored listings on search results pages. "Ads are still running, but advertisers are unable to make changes to campaigns easily," she said. She added that the day's difficulties weren't likely to have a long-term impact on campaigns. "Having it down for just one day probably isn't going to affect the larger scheme of things," she said.

This news story is important. I recommend that everyone read it.