On Tue, 22 Jul 2008, Paul Koning wrote:
  It says "5/32 shank".  Bizarre. 
Bits larger than 1/4" also exist.  But 1/4" is the defacto standard.
Harbor Freight has 5/16"
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95321
(including T40, T45, T50)
and my 40 year old hand hammer impact wrench takes 3/8"
  I recently ordered a security bits set from Harbor
Freight.  Stuff
 like torx with a hole in the middle... nice to have. 
Lately they've been putting their 32 piece set and 100? piece set on sale
frequently