From MPASM
Usage
This directive is used in the following types of code: absolute or relocatable. For information on types of code, see Assembler Operation.

This directive is not commonly used, but does provide an easy method of serializing devices. __idlocs can be read by a programmer. PIC18 devices can read this value at run-time, but PIC12/16 devices cannot.
For PIC18, I guess Darrel have the right ticket for that. Something based around the following
http://www.picbasic.co.uk/forum/showthread.php?t=4093