Microsoft has applied for a patent titled Mobile Communication Device Having Multiple, Interchangeable Second Devices. It describes a number of modules that would greatly extend the capabilities of a smartphone beyond what any single form factor could do.
The phone operates as the primary device. The second device could be a physical keyboard, game controller, extra battery, or even a second mobile handset with another screen. These modules are all wireless and can be used in groups. For example, if you want to play a game, just pick up the gaming module and set the phone down or lean it against something. Now you can use the gaming module similar to the way you would use a controller with a console games.
The idea isn't all that new, but it may be the right time for it. The tablet form factor hasn't outlived its usefulness by any means, but the limits of what it can do have been explored. Multi-touch greatly impacts how these large screens are used. Manipulating objects like pictures, Web pages, or just scrolling with your finger is much better than the old days of using a stylus, but entering data hasn't gotten any better. Software tries to aid in data entry, and while useful sometimes, it just as often creates amusing or embarrassing content. Sometimes there is no substitute for a good keyboard.
Having the keyboard as a separate module allows you to get some serious work done, but many phones can do that today. Where this gets interesting is you can have several modules working in conjunction with each other. You could use multiple modules together, say a keyboard with two displays. Or the phone would be the main module and could be used to display an article. A second module could be a screen with an email app on it, and a third module could be the keyboard you are using to type the email while referencing the article on the first screen. A fourth module could be an external battery used to keep the whole setup going for hours.
Microsoft only has one chassis spec for its Windows Phone 7 platform right now. It will be interesting to see if any of this makes it into subsequent chassis specs in the future. If anyone is capable of some cool hardware designs, it is Microsoft's key partner, Nokia. Think any of this will make it to market by 2012 or 2013?