One thing that's important to keep in mind:
As we make the transition to digital television, this "white space" we're seeing now will fill in, by the television industry without help from outside. Once the transition is fully complete, there won't be enough of it to matter.

Reason lies in the finer points of how digital television actually works.
In analog television, every frame is broadcast in it's entirety every so many fractions of a second. In the case of the system used in most of the Western Hemisphere, 60 times per second!
In digital television, on the other hand, there in an inherent bandwidth management technology built right into the system. In digital television, a frame is broadcast ONCE, and stored by the receiver. The next frame, only the things that have changed from the previous frame are broadcast, and the rest is in the Receiver's RAM!
Therefore, it takes far more digital bandwidth to broadcast say, a really fast-paced basketball game, or a NASCAR race, or a rock concert than it does to broadcast say "Meet the Press" or the "Weather Channel" In high-def, the situation gets worse.
More and more stations are now broadcasting second or third, or in the case of one local public station in my area, even fourth and fifth digital subchannels. 2 would be about the limit when you are doing something like a basketball game in high-def, maybe 3 if one is just text or weather charts but 5 is do-able when you show the types of content most PBS stations live on.
Once the commercial stations realize this, especially in a slow economy, they will no doubt see those digital subchannels as a source of increased revenue, and as a way to level the playing field with basic cable. Basic cable channels may well see it as a second path, in addition to satellite into smaller markets where the cable system is not getting the resources.
So, once we get past February 17, I look for this "White space" to essentially dry up before anyone other than the TV stations can do anything with it!

Click to read the article this is in response to.