Am I correct in understanding that what's actually detected and reconstructed by TEMPEST surveillance is the EMR having to do with screen drawing, and not the CPU and other internal components involved in processing data?

Cpu signal is the easiest to detect, but not the only one. Other possible sources of emissions are all kinds of communications cables. Unshielded RS-232 to the modem, possibly even the connections to the disk drives. I don't think the cpu itself emits much or if it does that it is easy to interpret it in any easy way, but I don't know for sure. The monitors are just easiest to detect, since for every pixel, the electron beam is turned on and off. These pulses are easy to detect, and if your monitor is synchronized with the target, a simple device could reconstruct the image, without any need for powerful computers to process the data.

If that's the case, would computers without CRT-type displays still be succeptible to TEMPEST surveillance?

It would be safe to say that they would be _less_susceptible_.

might be running a BBS without a video card inside or a monitor. Or laptops, such as a Mac PowerBook with an active-matrix screen?

Anyone that knows NSA's capabilities in these areas certainly isn't talking. It _may_ be possible to derive at some limits just by knowing electronics and the laws of physics. I don't know of any independent research team having done such a study. If it has been done, or is being done, I would like to know.