In therory, that should work.
but there seems to be a problem since 2007:
http://mail-index.netbsd.org/port-i386/2007/11/09/0001.html
# brconfig bridge0 add vr1 add ath0 up
brconfig: add vr1: File exists
# brconfig bridge0 add ath0 up
brconfig: add ath0: Invalid argument
# ifconfig ath0
ath0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 2290
ssid alix apbridge nwkey 2:"",XXXX, XXXX,""
powersave off
bssid 00:0b:6b:de:0a:74 chan 3
address: 00:0b:6b:de:0a:74
media: IEEE802.11 OFDM54 mode 11g hostap
status: active
inet 172.16.0.1 netmask 0xffffff00 broadcast 172.16.0.255
inet6 fe80::20b:6bff:fede:a74%ath0 prefixlen 64 scopeid 0x3
any ideas?