UPDATE: I’ve been getting a lot of traffic from Google on this post and I imagine many of you are looking for instructions. So I’ve embedded a video from YouTube user chr15b that shows you how to enable internet sharing on Mac OS X Lion.
One thing I happen to do every day when I come to work is to share my office’s internet connection with my phone. The connection comes in to my MacBook Pro through Ethernet and I then broadcast it to my phone over WiFi using the Sharing options in System Preferences. It’s a really easy way for me to cut down on my mobile data use when I’m sitting at my desk, since my office doesn’t offer a WiFi connection.
The unfortunate part of this happy little feature in Snow Leopard is that when I go back home and try to connect to my WiFi network there, I have to go back into System Preferences and turn off internet sharing so my Airport would go back to receiving mode instead of broadcasting. This was not a major annoyance, but certainly something that, after a long time of doing every day, got a bit tedious.
After recently upgrading to OS X Lion, I was delighted to discover that this process of constantly needing to toggle internet sharing is a thing of the past. Now I can keep internet sharing enabled, and OS X Lion detects when an ethernet connection is present. Lion turns on sharing automatically when ethernet is connected and turns it off when ethernet is unplugged. No manual toggling required. It just works.