As Apple Inc. launched its much-anticipated iPhone 4S Tuesday, its online retail site, Store.Apple.com was down from about noon to 3 p.m. Eastern time, says web performance monitoring firm Keynote Systems Inc.

Apple, No. 3 in the Internet Retailer Top 500 Guide, also launched on Tuesday a new mobile operating system, iOS 5, and iCloud, a new set of free “cloud” or Internet-based digital music files and photo services.

As typically occurs when Apple launches new products, it experienced a rush of traffic to its web sites. Keynote ran tests on Store.Apple.com from 10 locations throughout the United States and found a 100% failure rate from all locations, says Ben Rushlo, Keynote’s director of performance management. That means that visitors to Store.Apple.com during that period saw “error” pages indicating that the usual Apple e-commerce pages were unavailable, he adds. “If we see error pages from all 10 of our locations, we can assume this was affecting all site users in the U.S.,” Rushlo says.

Apple’s main corporate site, Apple.com, experienced similar issues on its home page from 2:45 p.m. to 4 p.m. Eastern time, he adds. For Apple.com, Keynote runs tests from 35 locations worldwide.

Apple did not immediately reply to a request for comment.