The upgraded service will feature e-mail caching to shorten response time, email message preview, ability to view more than one message at a time, quick search and drag-and-drop filing. Currently it only works in IE and Firefox, however if it doesn’t work for you, or you choose not to use it, the current version will remain an option.
The new Yahoo Mail won’t be out for a while yet, however it is expected to launch later this year. Lucky beta testers will see it in the upcoming weeks. Possibly by invitation only?