Yahoo! has no less than 4 iPhone-native applications in the iTunes app store. The latest is Yahoo! Messenger, which already was spotted on the original Yahoo iPhone application (our review here) as well as their mobile-optimized web site. This latest offering, though, is much more of a fully featured IM client for the Yahoo service.As of this moment,
Supported features include: IM and SMS sending and receiving, concurrent IM sessions (although not chat rooms as far as we can tell), emoticons and photos, and web URLs. There is also support for receiving notifications even when the app isn't running, due to the multi-tasking restrictions inherent in the iPhone OS. More details are on the Yahoo! Messenger blog.
Visually, Yahoo Messenger is very clean and easy to use. The standard virtual keyboard input area is customized to optimize screen space and allow incoming IMs to be separated from text you are still typing in. The app makes a sound when a new IM appears, or a buzz if the screen is locked but the app is running. If you exit out of the application without logging yourself out first, you will appear 'idle' to your friends so that they can still send you a message if necessary.
For the person that relies primarily on Yahoo! Messenger for their IM needs, this app should be all they require to get the most out of the service. We'd like to see a landscape keyboard supported simply because it is easier to type with. Take note that, (as CNet News also covered), the sound effects and vibration can't be turned on or off in the settings.
If you want support for more IM networks at the same time, there are a number of 3rd-party applications like Palringo (free) and IM+ (paid) that already support Yahoo Messenger among one of many other services, as well as maintaining your online status even when the app is not running.