This damn thread has gone everywhere, and I'm out of synch with what's been said and what I haven't read yet. But.....
Someone posted something about Kyllo in here, I think. The reason for the decision was that the use of infra-red was OK, and was not an invasion, but that the fact the light and heat were observed did not identify that property as being involved in any criminal activity.
The court said that it was possible to find many such innocent buildings with similar IR profiles, and there was nothing presented to the court which showed that, even with the IR profile, it was being used for criminal activity.
Now, I read that in here and not from any court document. But it sounds ok, explains the decision, and does NOT forbid LE from using the IR technique. Just needs some additional PC to make it work.
IIRC, the cops had an informant who told them about the grow site, the lights, etc. They kept the informant out of the judicial loop, IR'd the place and got a search warrant based pretty much on that.
IR is very useful in locating large fields of pot surrounded by other vegetation. And this can be used as the basis for a search warrant, since the profile comes from the MJ itself. Hemp and tomatoes give off the same profile as MJ.