What is AP Isolation Mode and why should I disable it on my router?

In the computer networking world, AP stands for Access Point, or in this case, a wireless access point. AP Isolation effectively creates a “virtual” network among wireless clients, in which each device is an individual entity that cannot communicate with other wireless devices on the same Wi-Fi access point. This configuration can be useful for public Wi-Fi hotspots to separate potentially malicious network traffic from other.

via What is AP Isolation Mode and why should I disable it on my router? – Customer Feedback & Ideas for Photosmith.

The tomato router has this option.  Other APs do not.