I know the use of a longer length seems counterintuitive to those of us who are used to "shorter is better". Shorter _is_ better, for analog cables.
Go to http://www.empiricalaudio.com. Scroll down to the link "Paper on S/PDIF cable length in PSOnline".
In a nutshell, this paper explains that signals are reflected internally in digital interconnects. Good DAC design can minimize these reflections but this is usually not done. In shorter cables, the reflected signal returns to the output very close to the rising edge of a wave, and confuses the DAC about the timing of the signal. The result is jitter. The longer length allows the reflection to return at a time when it is less likely to be confused with signal. The result is more coherent music.
(I apologize for any inaccuracies which may be found in the very brief summary above.)
You can also refer to Chris Ven Haus' Web site, where he specifies that 1.5 meters is optimum length for digital interconnects, and to UHF Magazine number 74. UHF tested different cable lengths and now do not recommend any length shorter than 1.5 meter.
UHF Magazine
I did my own tests and came to the same conclusion. I used 1-meter and 1.5-meter lengths of 75-ohm Apogee Wyde Eye (still a contender for best at its price point). We listened using my TEAC VRDS T1 and Apogee DA-1000E-20, and also a friend's Theta Universal and Kora Hermes. The results were unquestionably in favour of the longer length.
You could try a test yourself and see.
First of all, let me say that I'm not trying to prove anyone here wrong for believing that 1.5 meters is the 'best' length for digital cables, but I am trying to show that there certainly is not a consensus on the subject. According to Dan Lavry of Lavry Engineering, that Empirical Audio SPDIF article is "nonsense": http://recforums.prosoundweb.com/index.php/mv/msg/11678/0//0/