The BlackBerry PlayBook, unveiled by RIM today, has a 7-inch screen.

With all the buzz over Android and iPhone, it's easy to forget that Research in Motion's BlackBerry rules the U.S. smartphone market, with 40 percent market share, according to the latest comScore data. So the company's announcement today of a tablet device, dubbed the PlayBook, is worth more than a passing glance.

It's particularly interesting for its potential to provide a new competitor for Microsoft Windows on machines in that emerging space between traditional portable computers and smartphones. Microsoft has been pitching its Windows 7 PC operating system for tablet computers. But RIM's expansion into that space is another vote for the concept of tablets as outgrowths of mobile devices.

Even more than the iPad, the new PlayBook has the potential to strike at the heart of Microsoft's business by using RIM's existing inroads with enterprise customers, and remaining in their comfort zone. RIM gave this preview of its corporate pitch in the news release announcing the PlayBook:

Thanks to the seamless and secure Bluetooth pairing experience and the highly secure underlying OS architecture, the BlackBerry PlayBook is enterprise ready and compatible (out-of-the-box) with BlackBerry Enterprise Server. When connected over Bluetooth, the smartphone content is viewable on the tablet, but the content actually remains stored on the BlackBerry smartphone and is only temporarily cached on the tablet (and subject to IT policy controls). With this approach to information security, IT departments can deploy the BlackBerry PlayBook to employees out-of-the-box without worrying about all the security and manageability issues that arise when corporate data is stored on yet another device.

In related news, Amazon.com announced that it will launch a Kindle app for the PlayBook, showing how the new PlayBook will also double as a media consumption device. RIM says the device will be available starting early next year.