The instructions in http://www.live.com/wireless/unix-base-station.html
work very well with NetBSD 1.5, to make the machine an IBSS base 
station.  This isn't the same as a true base station, for reasons 
explained on that page, but clients expecting to see a real base 
station can talk to it without reverting to ad hoc mode.

Doing this requires using the -c and -q options to wiconfig, which the 
current man page says don't work.  (I'll file a PR on that.)  (The
page says -s instead of -q; I haven't investigated that yet.)  The 
trick, it turns out, is to use new enough card firmware.  The page 
says 6.04 is needed; I would note that a bug fix present in 6.16 solves 
the problem that a station with encryption off could talk in plaintext 
to a station with encryption on.... (For details, see
http://www.wavelan.com/support/software/description.html?id=291)