Do folding (or telescoping) cross-country skis exist and if so are they useable?
There is a folding ski system designed for snowboarders you can see here. I couldn't find any other cross-country folding ski available commercially.
Folding ski for cross-country is also patented under US Patent 6616170.
In addition to @Refineo's links, I was also able to find some other websites, but nothing really conclusive, or useful.
Similar to the MTNapproach skis: http://www.climb.co.at/index.php?lang=en
Russian Patent holder of folding x-country skis: http://www.vector-ski.com/collski_e.html
Backcountry style, but no longer available for sale: http://www.zigzagtour.de/index2.htm
Home-made folding skis: http://www.woodly.de/blog/?p=41