The bug is actually that ath_tx_start should check for multicast frames and 
  use a rate appropriate for control frames.

That sounds fine.  But what about packets to unicast addresses for
which there is no node entry?  In my environment, I would prefer to
send them at the lowest (most reliable, really) rate as well.

  Separately the rate control code in netbsd's ath driver appears to
  be old.  I recently split rate control out in the linux version of
  the driver so alternate rate control algorithms can be used w/o
  changing the driver (they are packaged as independent modules).

To compound this I'm using the 2.0 branch, not -current, and I import
that into CVS and then modify, so I'm triply behind (although only a
week or so behind netbsd-2-0 right now).