when does a 1/8" chain need replacing? I use a Park Tool device to check for stretch on my derailleured bikes - can the same tool be usefully deployed on a 1/8" 'track' chain, does anyone know?

If not, how then to check for wear?
The 1/8" refers to the width of the rollers in the chain (also the width between the side plates of the links). All bicycle chains have a 1/2" pitch (length of a link) which is ANSI standard.

Therefore your Park wear checking tool can be used with both 1/8" and 3/32" (derailed) chains as it checks for stretch along the length of the links.

All info on wikipedia:

http://en.m.wikipedia.org/wiki/Bicycle_chain