Yeah one brand of conductive-Ink pen is called "CircuitWriter"... Fry's stores usually carry it for about $10.00 (which is a total rip-off, but we are talking about fry's here.)... You may be able to find it cheaper on the web.
I have also seen people use a regular #2 graphite pencil to temporarily repair hairline cracks in traces on those "mylar printed circuits"....
