Here are some good refs I've found (I think the audioholics link, while talking about balanced connections, still does a nice job in explaining what's going on; ref'ed in the 990 thread I think):

http://www.audioholics.com/techtips/audioprinciples/interconnects/Balancedvsunbalanced.php
http://www.tubecad.com/march99/page4.html
http://www.hometoys.com/htinews/feb05/articles/ati/purebalance.htm
http://ftp.agilent.com/pub/semiconductor/morpheus/docs/diff_design.pdf pg 4 and 5
http://www.mcintoshlabs.com/mcprod/..%5Cdata%5Cbrochures%5CMC402br.pdf

There used to be something on Bryston's site too, but I can't find it at the moment.

And, if a balanced connection (interconnects) doesn't discriminate between even order and odd order distortion, why would a balanced design?