This is one of those things that a lot of people in the trade assume is always a code violation. Not necessarily. It is completly dependent on whether or not the circuit breaker is listed for one or two wire. Many 15 and 20Amp circuit breakers are in fact listed to accomodate 2 wires. Reference the link below from the web site of Mike Holt (who probably has more credability than me
http://www.mikeholt.com/technical.php?id=nec/unformatted/Jan_ECM&type=u&title=NEC%20-%20Question/Answers%20(1-4-2K)