Dell Vostro 3500 with Broadcom BCM4312 (as siba reports) accessing AP with WPA-PSK encryption enabled. AP is 2m away from notebook,directly visible. 

Loaded modules: 
if_bwn.ko 
siba_bwn.ko 
wlan.ko 
wlan_amrr.ko 
wlan_wep.ko 
wlan_ccmp.ko 
wlan_tkip.ko 
bwn_v4_lp_ucode.ko. 

Within first 1-2 mins after boot I get "bwn0: RX decryption attempted (old 0 keyidx 0x2)" errors at tty0. Pinging access point with 1000 packages gives 20% to 50% packet loss.

Loading Ubuntu LiveCD at the same conditions and installing offered 802.11 Linux STA driver gives 0% packet loss.

How-To-Repeat: grep -E "wlan|bwn" /boot/loader.conf 
wlan_load="YES"
wlan_wep_load="YES"
wlan_ccmp_load="YES"
wlan_tkip_load="YES"
wlan_amrr_load="YES"
if_bwn_load="YES"
siba_bwn_load="YES"
bwn_v4_lp_ucode_load="YES"

grep -E "wlan|bwn" /etc/rc.conf
ifconfig_wlan0="WPA DHCP"
wlans_bwn0="wlan0"

cat /etc/wpa_supplicant.conf 
network={
	ssid="ittomsk.ru"
	psk="******"
}

ping 192.168.0.1 (AP address)