Microsoft this morning released a Windows Live Messenger app for iPhone, bringing its widely used instant-messaging program to Apple's popular mobile device.

The new app (iTunes link) comes about six months after Microsoft's release of Bing search for the iPhone, and a couple weeks after Microsoft and Apple exchanged some surprising public compliments -- making it clear that Google has replaced Microsoft as Apple's Enemy No. 1, while underscoring the reality that Microsoft's online services need to fully embrace mobile platforms that compete with its own.

I've installed Windows Live Messenger on my iPhone and started running it through its paces, and so far it looks pretty good. Basic features include the ability to IM with friends, of course, but beyond that it promises to deliver push IM notifications when the app is closed, if someone sends a message.

It also offers direct access to Hotmail, as well as Windows Live social networking updates, which can be set up to include updates from Facebook and other services. The photos section of the app lets users create albums and upload images.

I like the ability to click on buddy icons from anywhere in the app to quickly access their profiles, start an IM conversation, or send them an email from the iPhone's standard email client, even using another email service.

However, the experience using WLM for iPhone has also opened my eyes to the problem with IM on mobile phones in general. I'm not a big user of mobile IM, and despite my prowess with the iPhone keyboard, I'm having a hard time keeping up with my IM buddies who are using standard computer keyboards.

Specific to the Windows Live Messenger app, early complaints in the iTunes store include the inability to hide offline friends or change status. [Update:As Nathan Novak points out in the comments, you can actually change your status by clicking on your own user icon from the app's Social tab, which brings up a flyout menu that lets you click on your current status to bring up another menu that lets you change that status.]

The release coincides with today's expected debut of iOS 4.0, the name for the new version of the iPhone operating system. Requirements for the WLM app say it's compatible with iPhone, iPod touch and iPhone going back to iOS 3.0.

Microsoft has been planning a Windows Live Messenger iPhone app since last year, as reported by Neowin in December. I first spotted today's news on LiveSide.net, which got it from the Redmond Pie blog via LiveSino.net. Also see TechCrunch and The Next Web.

If you use Windows Live Messenger but not an iPhone, here's a handy (albeit outdated) guide from Microsoft explaining where and how to get the service on Android, BlackBerry and other devices.