Well actually the result of "running out" of addresses is undefined and up to the individual hardware manufacturer, so theoretically they could do whatever they want. A high-traffic WAP might want to simply boot the oldest lease if they can't assign more than a certain number of addresses.

Keep in mind if aa client wants an IP, it is supposed to do DHCP discovery every time it connects to the network, even if it's lease isn't up. See http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Prot...