Active Thunderbolt Cables Are Hot Cables
You might not think that an external connectivity solution would have thermal issues to worry about, but Thunderbolt is quite literally a hot technology.
An infrared image of where a Thunderbolt cable plugs into our motherboard reveals temperature readings above 110o F, even when downstream devices are idle. Active, we see the cable exceed 120 degrees.
Of course, those temperatures are a result of the active Thunderbolt cable, with two Gennum GN2033 chips on each end. As information moves through the cables, the hard-working data transmission chips heat up and cause those more extreme readings.
Not surprisingly, more space-constrained applications, like our 13.3" MacBook Pro, demonstrate even more alarming thermal properties. In the shot above, the Thunderbolt cable is the one up in the 120o+ range. Next to it, on the left, you can see a FireWire 800 cable. On the other side, there's a USB 2.0 cable. Although those two interfaces look like they're giving off heat as well, they're actually being warmed by the Thunderbolt cable. Fortunately, only the ends of the cable heat up; everything in between stays cool.
Those lofty temperatures aren't a problem if you're using a mini-DisplayPort adapter. The display signal is already demuxed by the controller before it hits the adapter.
So, in comparison to USB and FireWire, Thunderbolt cables get pretty darned hot. But the heat dissipated only causes the plug to become uncomfortable to handle for any significant length of time, and they won't burn you (the same conclusion we reached about gaming on an iPad 3 at maximum brightness).