Uma pergunta muito interessante, então eu pesquisei por aí. Isso pode não ajudar, mas encontrou esta resposta, citando isso:
You can try this, although I can't promise it will work every time.
- Open Control Panel\Network and Internet\Network Connections
- Right click on your 3G adapter and select properties
- Highlight Internet Protocol Version 4 and click the properties button
- Click the advanced button.
- Un-check the "Automatic Metric" check box
- Enter a value of 10 in the "Interface Metric" box.
- Click OK all the way out.
Now repeat the same process with the WiFi adapter, but give it a metric of 1.
What your telling Windows 7 is that given two active connections to the internet, use the one with the lower metric...in this case WiFi. This only impacts situations where both connections are active. When only one is active, it "should" use that one, regardless of the metric. I say "should", 'cause you know how it goes with windows.