Via Louis Gray (via Buzz)
Tasker's an automation app for Android. You can define a set of criteria that, when met, trigger an action - loading an app, presenting a menu, changing a system setting, etc. For this particular scenario, I just wanted to have the phone turn on airplane mode (but keep wifi and bluetooth on) whenever I'm home. Tasker made this a trivial task - and the battery at the end of the day is now often north of 40% where it was previously empty by day's end.
There's a lot that Tasker can do for you - check out the example profiles included in the Tasker wiki. Each profile is described in detail, and includes a download link so you can load the profile directly into Tasker. For around $6, it's a great deal. (You can download a 7 day trial here, if you want to kick the tires before buying.)