I just saw this posted on Slashdot, and it certainly appeared applicable to PIC development. It is an extremely light-weight library that implements pseudo-threading. It was specifically designed for embedded use, so it should be usable on most PICs. It only requires two bytes per thread.
The library is written in ANSI-C, but it should be possible to port (if not port then at least borrow the concepts) to PIC ASM because it is so small. It certainly could be useful for more complex software implementations.
https://www.sics.se/~adam/pt/
I searched the forums and couldn't find any posts about it, which I assume is because it is relatively new (first news entry on the website is February 2005).
Dan East