High Resistance PCB materials - reasons behind it?
As Russell said, all pure PCB material is high resistance.  However, there are different processes.  Not all PCBs are created equal.  I worked for HP a long time ago, and one of the divisions had a special "high Z" process.  There were specs for Ohms per square, but I don't remember the numbers.  Theirs was significantly higher than what you'd get ordinarily.

If I remember right, the main difference wasn't so much the material but its handling and particularly its washing.  They may have used a special solder mask.  I do remember those boards were more shiny on the surface than ordinary boards.  There was a paper on that stuff, and what I remember most of it was about getting rid of stray ions by more careful washing procedures.  The raw material is a very good insulator.  Just about all surface conductivity is the result of impurities (dirt), particularly ions left over from the other processes the PCB is subjected to during manufacturing.  This also means you have to keep the board clean and handle it carefully after manufacturing else it won't stay high Z.  One fingerprint in the wrong place and you've wasted your money on the high Z process.

As for why you'd want it, it is for high impedance signals.  Leakage currents can add up fast on PCB traces.  10 M&Omega;/square may sound high, but if you have two tracks running next to each other for 10x the spacing between them, they are essentially connected with a 1 M&Omega; resistor.

The usual and more reliable way to deal with very high impedance signals is to use guard traces.  The high impedance trace drives a unity gain buffer amp.  The trace into that amp is then surrounded by the buffered output.  Since that is at the same voltage or close to it, any resistance to the guard trace won't cause current.  You have to be careful in layout to make sure the guard trace properly surrounds the high Z trace everywhere current could leak accross the PCB surface.  This guard trace scheme allows boards to have lower surface resistance so makes things more reliable.  Sometimes when even that isn't good enough you look into special high Z PCBs.  After that it gets big and expensive, like special glass insulators and the like.  You don't go there unless you really need to, like you're in the G&Omega; range.