Would a plan year beginning 10/1/2011 be subject to the 2011 or 2012 limitations

What limits are you talking about? If you mean the compensation limit, you would work off 2011, i.e., $245,000. If you mean the 415 limits, that would depend upon the "limitation year."

Just to expand upon jpod's answer - the 415 limit is for the calendar year in which the limitation year ends. So assuming your limitation year ends in 2012, then you'd use the 2012 limit.