Last year or so we quickly put together a script (with the help of kind folks here) which checks first for an Ethernet connection, if not then the wifi script is initiated. Since then this method has functioned seamlessly (well, except for a few password glitches) otherwise I forget of it's existence. A script to accomplish this task is easy to create and well worthwhile, it has simplified connecting to the network from my notebook at whatever location I'm at. though I seem to remember making a few changes along the way, now my only concern is entering a password if one is required, Is something like this what you're thinking of??
the wifi.tcz extension has been invaluable to me while traveling.
edit, on reflection I had to make a couple of edits here. hope it reads better now