Unobtrusive sensor for detecting touch on a plush toy?
My first thought was conductive thread too...

There are a lot of people who do embedded circuits into clothing that might provide some insights. Wearable Computing yielded some solid search results for me.  Lilypad Arduinos get stitched into clothes with conductive thread, looking at those designs online might get your further down the rabbit hole.

I've seen a few designs out in the webiverse for winter gloves that insert conductive thread patches into the finger tips to make capacitive touch screens work for folks.
I'd think that heading in a similar direction might meet your needs.



A few links which may trigger some ideas/answers:

http://www.media.mit.edu/wearables/

http://hlt.media.mit.edu/?p=1283

http://cabinet-of-wonders.blogspot.com/2007/12/lilypads-and-wearable-electronics.html



http://www.instructables.com/id/Making-A-Glove-Work-With-A-Touch-Screen/

http://en.wikipedia.org/wiki/E-textiles

http://cabinet-of-wonders.blogspot.com/2007/12/lilypads-and-wearable-electronics.html