Contrary to popular belief, energy in the infrared spectrum is not necessarily heat. For our purposes, lets say that there are 2 portions of the electromagnetic spectrum in the infrared region; Reflected IR and Thermal IR. (Reflected IR being in the .7-3.8 um wavelength and thermal IR (heat) is from 3.8 um to approx 800 um). Forward looking infrared systems (FLIR) can "see" images anywhere from 3-5 um and 8-12 um (the atmosphere blinds the systems from 5-8 um). Pot plants do have a specific "signature" in this spectrum(and other spectrums), as do all plant species. While this might sound disconcerting, the cost of this type of system is very high, and they do not routinely fly looking for small grow ops. They look where they already have suspicion to look. Also, the resolution on these systems is also not nearly as good as standard photography, so if one were to keep their grow sites small (2-3 plants is what was previously suggested, and that sounds about right to me, but you could probably get away with a few more.) the sensors on these might not register the plants, or if it does, the person interpreting the image will think of it as an anomaly in the image or something minor. To make a long story short, they wont bother you if you keep everything small.
As a word of advice to all growers (indoor and outdoor) there are many ways one could be busted, and FLIR is only one of the many. Growers tese days have to worry about a multitude of remote sensing systems such as across track scanners, TIMS (Thermal infrared multispectral scanner), Along track scanners, such as Multispectral electro-optical imaging scanners (MEIS). One of the most dangerous sensors to indoor growers is the VANSCAN system, which operates from the side of a van and can take images of houses in the thermal IR spectrum (ie. it can detect heat escaping from your house). So be careful!