GPS (atleast the commercial versions) use very weak signals and usually require an unobstructed view of the sky to lock onto the various satelites. Even so, commercial accuracy is not all too good (not sure of the exact number, but I found this
here:
Quote:
Improving the GPS
Since 1990, when the U.S. government made available the Global Positioning System (GPS), there have been calls for correcting the built-in error that the Department of Defense (DoD) required when the system was launched. The error, called Selective Availability (SA), was installed by the request of the DoD to deter the system from being utilized by terrorists or other potential enemies. The DoD intentionally varies the timing mechanisms aboard each GPS satellite in order to limit the accuracy of the system. Without SA the commercial GPS accuracy would be about 20 meters. With SA, the system's accuracy can degrade by as much as 100 meters. There have been continued calls for removal of SA and the government has stated it will phase it out over the next few years. However, in the meantime, users of GPS have found methods for overcoming the error. Entrepreneurs have improved GPS accuracy by using differential techniques to measure and correct the artificial error. And while the removal of SA would increase the accuracy of the system, at this time many users desire even greater accuracy. Thus, many GPS users say the need now is for the development of better differential techniques with or without the presence of SA.
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The point of this is that you're not going to get anything more accurate than a few (dozen) meters - inside a building, even if you do get a signal, it would be a few hundred meters. For travelling around town, that would fine. For travelling around a FIRST field, it's completely useless.
As for other things, I don't know what else you could use them for. The only use I can possibly imagine (without broadcasting anything back) would be to create some kind of secondary remote control. This of course brings a whole different set of rules into question, mainly the ones about participation from the audience and how it's frowned upon.
Aside from GPS (which turns out to be useless for our purposes), I can't really think of any reason that is legal or any useful purpose. What specifically did you have in mind?
Speculation:
Since GPS signals are blocked by buildings, anyone think it's possible to create a 'relative' gps system - like set up transmitters around the arena and create some triangulation technique to get relative motion from those?
Speculation 2: Actually, it would be 1000x easier to do what WildStang did this year to keep track of relative motion.