Yahoo's push onto mobile devices is inching forward, with a new version of Yahoo Mobile being released in in a limited beta. Encompassing more than just messaging, the new software has a variety of functions included such as email and social networking support, scheduling, mapping and more. Though not fully available to the public, Yahoo has mentioned that their new software will be released in beta form to the world in the second quarter of this year.
There are ways to connect with YIM servers on a mobile through the use of several different clients. None of them are "sanctioned" Yahoo-developed applications, though many are free and exist across just about all smartphone platforms that are out today. Yahoo's selling point will be the higher level of integration, with access to more than just messaging services. So long as they can keep the service free, they might have a lot of success, otherwise I don't see it working out.