I've seen this discussion somewhere else, possibly in the IAEI magazine. The consensus of the discussion was that if there were no markings on the breaker or the enclosure, the line could be connected to either end. This is in agreement with what is in the UL White Book (DIVQ) last sentence of the Installation paragraph "Line and load markings on a circuit breaker are intended to limit connections thereto as marked."
Asking the installer to identify which end is the line connection is an excellant idea and sounds to me like a possible proposal for the 2011 NEC.
Tom
