You could always follow the link I provided and verify for yourself. In fact I recommend it highly, so you can see the bullshit close up.
I know that this isn't a mythbusters proof of flir limitations, but in season 9 they did a thermite vs ice myth. In that myth they used a thermal imaging device to read the temps. Heres a link to that particular myth.
http://www.youtube.com/watch?v=BnHR4cMXiyM
The part that is revealing to me about thermal imaging is at 2:11 through 2:15, the imaging is on and you get to watch three people walk back too and behing the plexi blast sheild. When they walk behind the sheild (which is thick clear plastic) they all disappear to the thermal imager.
Before anyone says that it was because it was a special blast sheild understand that their "blast sheild" is simple a thick piece of clear acrylic. The mythbusters have explained on many episodes what the "blast sheilds" are and how they work. They have no special thermal properties and no insulating properties.
Also at 2:50 there is a side by side with thermal imaging on one side and a regular camera on the other. You can see the thermite is in a metal container on the regular camera but the container isn't visable to the thermal imager. The thermite is ignited and the thermal imager picks up the heat and flame above and below the container but never sees the heat inside the container. I think that says alot.
Also check out 6:36 through 6:45 where the extreme heat transfer throughout the whole area and the thermal imaging sees everything as red. You can see the process of heat being rapidly transfered to its surroundings,
Also worth noting is that thermal imaging CAN also SEE cold things.
I understand the concerns about thermal imaging, and believe that the public has been led to believe that thermal imaging can see through anything. The fact is that at this point thermal imaging can ONLY see heat signatures that are UNOBSTRUCTED.
The reason that the guy in the shed was visable was because of the transfer of heat from his body to the wall. The FLIR saw the heat transfering from his body through the wall because he was leaning on the wall. You can hide a flame behind a piece of glass as long as the flame doesn't transfer its heat to the glass.
Now all that being said. If you are transfering ANY heat through your exterior walls AND it it more than the ambient temp of the wall the heat WILL register on thermal imaging. This was demonstrated to an extent in the FLIR video posted earlier when you see residual heat PRINTS. Once when a person purposely put their hand on the wall and then used a finger to draw a circle, and once when the guy ran to the tree and put his hand on the font side before going behind the tree.
Hope I was clear enough; the mythbusters video should be convinving enough to dispell most misinformation about thermal imaging.