Comparing ident's, the only change in even vaguely relevant files is

  -      $NetBSD: ath.c,v 1.59 2005/09/13 05:50:29 martin Exp $
  +      $NetBSD: ath.c,v 1.60 2005/10/14 00:26:45 gdt Exp $

  I don't think this change is the cause, but I'll try backing just it
  out, but probably not until the weekend. (ident diffs for irrelevant
  files happily provided on request.)

I'm running with this change (write wep keys on power up) in current
from 20050926 with extensive local changes, and it works (IBSS mode,
40 bit wep) as well as ath ever did.  I have problems with failed
transmissions and blame rate control (it stays at 5.5 Mb/s instead of
dropping to 2) but haven't switched algorithms or investigated yet.
But I haven't use AP mode with ath in the last few months.

You might try forcing a low rate on the AP to see if this clears the
problem.

-- 
        Greg Troxel <gdt@ir.bbn.com>