ifplugd fails to bring up wireless interface

[Can-Hua Chen <chencanhua@fudan.edu.cn>]

I told ifplugd that my wlan interface is a hotplug one.
And it fails to bring wlan up automatically.
Below is my "/etc/default/ifplugd":
  INTERFACES="eth0"
  HOTPLUG_INTERFACES="eth2"
  ARGS="-q -f -u0 -d10 -w -I -b"
  SUSPEND_ACTION="stop"
PS. ethernet interface "eth0" works fine with ifplugd.
Any hints? Thanks.