Today I discovered how. At least for my home network, and my particular router. But it may be valid for others.
My router is a Linksys WRT160N.
Enter the router's (usually at 192.168.1.1 for Linksys and 192.168.0.1 for others) GUI.
Go to Status->Local Network. Clic on the DHCP Client Table. (Other routers will have a different place for it, but they will have a DHCP Client table):
In my case it has 6 columns:
Client Name | Interface | IP Address | MAC Address | Expires Time |
the sixth being a column with Delete buttons in each row.
Again, in my case, under Interface, I get Wireless-G for my 3rd Gen iPod Touch and Wireless for my iPad 2.