Although it is a little late but Yahoo! at last released their very first Android apps for Messenger and Mail, a Yahoo! search widget for your home screen and some HTML5-optimized sites for the iPhone.
The Messenger App: It allows you to chat with your Yahoo!/MSN friends, and can run in the background (with push notifications) for that always-on availability. If you live in Canada, Indonesia, India, Kuwait, Malaysia, Philippines, Pakistan, Thailand, United States, or Vietnam, you can also use the app to send SMS to friends, which will appear in the current conversation. And it also supports emotions, because it ain’t IM without emotions.
The Mail App: It is also a no-brainer, and allows you to check, search, organise, and send mail from your Yahoo! email account, as well as download/upload attachments. It can use both your Yahoo! address book, as well as the one on your phone, so you should be able to find the email address you need. And, like the Messenger app, it also supports push notifications and emotions.
The Search Widget: And, finally, the Search Widget does pretty much everything that the Google version does, including local search, voice search, and suggestions as you type.
Note: As you can see in the screenshot, there is a button in both of the apps that allows you to switch between Messenger and Mail.