In a CES keynote or two of year's past, Bill Gates talked about a future where we'd be able to share certain computer experiences across multiple devices. Most commonly that means the PC, phone, and TV. The idea of being able to start a task on your PC and finish it on your phone or on the TV in the living room is a truly compelling idea.
With Windows Phone 7 series and the recent demonstration of a true multi-platform game that you can start on your computer and continue on your phone or Xbox (thanks to common XNA development), we may start to see some killer ways to leverage this concept. The most common example is in gaming, but there are many other scenarios where a multi-platform approach would lead to some cool possibilities. Add the Courier to the mix (due to be released later this year), and the potential expands. Imagine taking notes on your Courier in a meeting. The notes are sync'ed to the cloud, and on your way to lunch, you pull out your phone and review the notes you took earlier. Then, you get back to your laptop, pull up the same notes, and use several quotations in a PowerPoint presentation you're working on.
Another example of how multi-platform development could enrich our lives is through entertainment. Start a TV show on your desktop, continue where you left off on your phone as you commute to work, then finish the show on your Xbox in your living room when you get home in the evening.
Also consider looking for a new restaurant to try. You use an application on your PC to narrow the choices. Then, you get in the car, and your phone knows which restaurant you've chosen, and it launches the navigation software to help you get to your destination.
You can already do a lot of these things through different applications and protocols. But the idea behind multi-platform development is that the apps are programed in common languages, which allows for consistent interfaces and experiences, whether you're looking at a 3.8" screen or a 42" screen.
What killer multi-platform apps or tasks would you like to be seen made possible?