Ultimately, the TSA agent who inspects your bag can arbitarily prohibit anything which he doesn't like the looks of (and "last week, they let this same item go," is no defense). Obviously, he has to ultimately answer for his decision, so he's not going to be unjustifiably outrageous. I have never heard of an "ordinary" flashlight drawing much attention (which has always puzzled me since there is a company here in Oregon which makes an obj9ect that, as far as can be seen externally, is a 2-AA Maglight flashlight but which "lights up" in an entirely different way... a .32 caliber way. I'm told that their single largest customer is the US Marine Corp which uses them to train embasy guards.) And one could certainly conceal an adequate amount of high-explosive in the body of a small flashlight to do some damage. But, even so, they don't seem to draw attention.)
I have, however, heard of people having trouble with some of the more agressive "tactical impact device"-style flashlights and so I would suggest not carrying anything which obviously crosses the line from flashlight into weapon.
