Given recent events in Japan, this got me wondering. Is it possible to determine radiation levels reliably not having Geiger counters near the possible radiation contaminated zone? According to wikipedia the Chernobyl disaster was first (other than Soviet Union naturally) detected by Swedish via radioactive particles found on clothes of nuclear plant workers. Surely more efficient ways should have been developed by now.
Short answer: no.
Longer answer: No, excepting neutrinos none of the products of radioactive decay has the penetrating power to pass through the atmosphere, and neutrino detection is not something we can do from satellites.
To elaborate, the immediate products of radioactive decay are (some set of, depending on the decay in question) fission fragments, electrons, positrons, alphas, neutrons, photons (gamma rays) and neutrinos. Plus the remnant nucleus. The only secondary product which might be interesting is Cerenkov light.
The electrons and positrons will travel a number of cm in air (at ground level). The gamma might go a few meters. The heavy stuff has no penetrating power at all.
Even if lofted to the top of the troposphere, there is just too much air in the way.
Cerenkov light will, of course, go through a lot of atmosphere, but you'd be looking for a pale blue glow against the general light background. For dispersed radionucleides (i.e. contamination), the intensity will be awfully low.
N.B. I too have seen various TV show and movie where some character from some agency says "We can track the radiation with satellites!". I believe this to be misinformed babbling of desperate script writers.
I hear that the WC-135 Constant Phoenix has recently been deployed to monitor radiation in the air around Japan.
The Vela satellites included gamma ray detectors, which accidentally discovered astronomical gamma-ray bursts (GRBs).
Since these satellites can detect ICBM launches from the ground and nuclear bomb detonation on the ground, and such detonations produce lots of radiation including gamma rays, and these satellites include gamma ray detectors, it would be easy for people to (incorrectly) jump to the conclusion that these satellites detect gamma rays emitted from the ground.
As dmckee and Martin Beckett pointed out, our atmosphere is far more than thick enough to convert practically all cosmic rays into gamma rays, and practically all gamma rays into visible light and muons. That's why all gamma-ray telescopes and X-ray telescopes are designed to be placed outside the earth's atmosphere.
The gamma ray detectors on those satellites can't detect gamma rays emitted by things on the ground; apparently those detectors were originally designed to detect gamma rays emitted by nuclear bombs detonated in space. (Has that ever happened?)
Stuff that happens on the ground is detected by the other sensors on those satellites.
As far as I can tell, satellites pretty useless for measuring radiation levels around nuclear reactors. Radiation levels high enough for them to see (indirectly -- gamma rays converted to visible light) are also bright enough for humans to see, and only nuclear bomb detonation gives radiations levels that high. The radiation levels that a damaged nuclear reactor could produce in a worst-case scenario -- lethal but not high enough to indirectly produce enough light for the human eye to see -- that light is also far too dim for these satellites to see.