At the Web 2.0 Expo in San Francisco yesterday, Yahoo announced that it is completely redesigning the underlying structure of its services, including Flickr and Yahoo Mail. At the same time, it plans to reveal the technical aspects of this redesign so that programmers can build features on top of them and combine them in novel ways.

“We’re in the process of rewiring Yahoo from the inside out and will open up all the assets of Yahoo to developers across the Web in a way we’ve never done before,” said Ari Balogh, Yahoo’s CTO, in a keynote.

One of the goals, said Balogh, is to make use of the social-network data that resides on the company’s servers. “We’re going to make the consumer experience at Yahoo social throughout,” he said. For instance, instant-message capabilities could be incorporated with Flickr.

If any Web company has untapped stores of social data, it’s Yahoo. Consider that the company has one of the most popular Web mail services. (We wrote about Yahoo’s vision for a more powerful, social inbox here.) Nearly 100 million people use Yahoo Messenger. Flickr is one of the top photo-sharing sites on the Web. And Yahoo Answers, a question-and-answer service in which people pose questions that others answer, has been a huge recent hit. But until now, Yahoo has only flirted with the idea of unifying these services, the data, and the inherent social connections that exist within them.

Yahoo’s new openness is surely exciting to many people who value open software. Indeed, open platforms invariably lead to innovations that are impossible for companies, often mired in bureaucracy, to produce on their own. To be sure, Yahoo has a history of embracing this sort of openness. According to Balogh, Yahoo claims 200,000 developers that already work on some of its services, some of which have been open for years. And Flickr is the second most popular Web service in the world to tinker around with (as measured by the number of times its application programming interface, or API, has been downloaded).

But I’m skeptical that Yahoo will be able to pull off a project of this scale. The company’s massive layoffs and the buyout offer from Microsoft show that it’s struggling. Yahoo may have great technologies, but it also has a habit of sitting on projects that are never released to the public. When contrasted with Google’s constant flood of new software and services, Yahoo’s sluggishness is shameful. At this point, it’s unclear if the company will be able to manage a project with the complexity of connecting and opening all its Web services.

Another concern: a platform needs to be compelling for programmers, who are, as much as anyone, swayed by intangibles such as buzz. When Facebook opened its platform, programmers leaped at the chance to build applications, with hopes of riding on the coattails of the social-networking site’s rising popularity. Yahoo will need to fight its image as an also-ran in order to court programmers who can make compelling new software. And it must also make its platform seamless and easy to use. Otherwise, Yahoo’s act of opening up its services will look less like a bold business move and more like just another good intention.