For years, I had problems with the default route going away with a wi0 
card.  See, for example, http://mail-index.netbsd.org/netbsd-users/2003/12/17/0000.html

I now have a new laptop, with an ath0 wireless card, and I'm seeing the 
same thing.  It isn't a signal strength issue; I've seen it with the 
laptop in the same room as the access point.  I saw it 20 minutes ago; 
wlanctl says there's a received signal strength of 53.  Carrier didn't 
drop, according to ifwatchd.  I don't know how good 53 is on an 
absolute scale, but with the AP about 1.5 meters away I see 53-64, so I 
assume it's pretty good.  

So -- does anyone have any idea what's going on?

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb