I think Zimin has been hanging around the "phull volume" guy to much.  I 
think what he is trying to say is that you are calculating the historical vol 
correctly but to compare this (historical vol) to an implied vol from either 
the marketplace or as calculated by yourself, there will be an underlying 
assumption of a calendar year as the basis for option maturity. Therefore you 
would have to use the sqrt of 365.25 in order to make a comparison to the 
historical vol.