Note that Brett has already mentioned backward compatibility issues, but suggested that "[adding tz_aware argument may provide] a transition plan to make utcnow() always return a timezone-aware datetime object." [msg106413]

I would say, lets leave utcnow() alone.  It is ugly enough without a boolean argument.  I don't see how datetime.now(utc) can be too error prone.  I think Brett's comment about a stupid developer was about passing tzinfo instead of bool to utcnow() and that I agree makes no sense.