Need to connect to specific wifi point as soon as it is in range. If it is out of range, then system must stay connected LAN. In case, any wifi-point is in range, there must be auto-switch.
Does the LAN and Wifi AP belong to the same network? If yes, then you can create a bonding device between LAN and Wifi. Bonding is done usually on servers to connect them to different network switches for redundancy and/or performance and kernel chooses which link to use. But it can be used also for your purpose (in fact, one of my colleagues uses it all the time at home - he works on Wifi, but when he want fast download speed, he just plugs in a cable and the download continues over LAN without any interruptions).
Here is a Bonding HOWTO: bonding.txt