Detect a device's presence on the network

I have a WNT3500L running Tomato Firmware v1.28.7500 MIPSR2Toastman-RT K26 USB Ext. What I'd like to do is detect the presence (or absence) of my iPhone on the wireless network. I've assigned a fixed IP to my iPhone. I've seen entries in the router's logs when my iPhone connects to the wireless but there was no entry for when it disconnects. Is there a way to log this event? Or maybe there is a better way of doing this, like a cron job that could check the device's presence on the network? Thanks for any help and suggestions.