My configuration is using a wireless bridge and it doesn't work after I shutdown the computer for a while, however if I turn on the computer and shutdown it, WOL works fine.
It seems that I have to setup a static arp with the MAC address of the wireless bridge I am using (TP-Link WRN700N).
under DD-WRT, it is something like that:
arp -s 192.168.2.50 (Computer's static IP) xx.xx.xx.xx (wireless bridge's MAC address)
update: well this still doesn't fix the problem, in my case My PC was connected to a switch which is connected to the wireless bridge, after remove the switch, I am now able to wake the PC even after it has been shutdowned for a long time...I suspect it it the arp cache in the switch expired and caused the issue.
The Ultimate Wake-on-LAN (WOL) Guide:
Tip: Wake on LAN/Magic Packet information collection and check list