ATH driver doesn't associate with turbo.

[randy at psg.com (Randy Bush)]

it seems mode 11b is preferred.  if i boot in the presense
of both kinds of APs, it seems to choose the 11b.  i can
then say

   ifconfig ath0 mode 11a

and it will happily switch to 11a

( and to <bleep> with turbo.  real wireless users eat turbot
  for lunch :-)

randy