If you are testing Yahoo! Mail Beta right as well as Internet Explorer 7, then you might already know that there are some conflicting issues. Yahoo! Mail Beta is set up to only allow certain web browsers to use its services to help them ensure that the users get the best experience. They have not setup Internet Explorer 7 as one of those browsers yet. If you are using Internet Explorer 7 Beta 2 then you can change your User Agent so that IE7 will pretend to be IE6 SP2 when identifying itself. This is just a simple registry import brought to you by FiddlerTool.com! All you have to do is download it from the link below and run it.
Patch: Registry Fix To Change IE7 User Agent To IE6
News Source: The Unofficial Yahoo! Weblog