What card are you using?
What chipset is it and which driver does it use?
Did you bring up the interface before the other comands (ex: ifconfig ath0 up)?
What BT version are you using?
i have tried
dhcpcd ouputsCode:iwconfig ath0 essid <essid> key <key> dhcpcd ath0
Code:root@bt:/# dhcpcd ath0 err, ath0: timed out err, ath0: lease information file `/var/lib/dhcpcd/dhcpcd-ath0.info' does not exist warn, ath0: using IPV4LL address 169.254.167.44 root@bt:/# dhcpcd.sh: interface ath0 has been configured with new IP=169.254.167.44