I spent a long time today struggling to work out why DNS lookups were failing on a MacBook Pro while connected over Airport WiFi, but not when using an ethernet connection to the same router.

It turns out that although the Airport connection looked perfectly normal, when I went into System Preferences -> Sharing, it showed that Internet Sharing was enabled. The AirPort status icon in the menu bar did not show that Internet Sharing was enabled, but sure enough as soon as I unchecked the box everything was back to normal.

Apple Leopard fail!