This isn't a new idea by any stretch, but I'm hearing a lot more buzz about it these days. The core concept is that if you create your mobile app first, it will focus on the essential features.
A secondary benefit is that mobile will probably surpass desktop use (particularly when it comes to search) in the near future.
Luke Wroblewski's blog entry on the subject is short and clear. If you want a bit more depth, consider skimming this presentation by Tyler Tate, which focuses on search applications.