One of useful application of UAVs (multi rotors) is in research area. low speed, low flight altitude and high spatial resolution of acquired imagery are some of UAVs capability in research studies. One of restriction in using UAVs is large numbers of imagery taken in each flight (and need time to mosaic imagery). By increasing flight altitude this problem is solved.
It depends on the country you work (US I guess for you). In New Zealand we can't go above 300 ft and we need to notice the local aviation. Rules are in the process of becoming a bit more strict. Civil Aviation is much more relax in Antarctica, where the plane activity is reduced.
In Europe, there are specific restrictions for inhabited areas and flying corridors and airports...
For the US, a good start would be to contact the Federal Aviation: http://www.faa.gov/
.... and there are variations in the rules, if we do recreational, teaching or consulting flights...
You can take a look at: A survey on position-based routing protocols for Flying Ad hoc Networks (FANETs) in which different restrictions about the use of UAVs have been referenced.