I've been having problems using dhclient with media or mode statements 
in my interface specifications.  For example, I'm at the IETF meeting, 
and if I have

interface "ath0" {
	  media "nwid 'ietf63' -nwkey mode 11a";
}

in my dhclient.conf file, it doesn't get a lease.  Deleting that line 
and setting 'mode 11a' manually makes everything work.  I've had 
similar problems with wm0 -- saying "media autoselect" made it not work 
on the wired ports.  But setting manually made everything work well.

I'm running -current, kernel and userland, from about a month ago.  The 
only other options I have set that might be relevant are these:

timeout 60;
retry 30;
reboot 10;
select-timeout 5;
initial-interval 2;


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