Last week, when I was offline, Google announced a new AdWords feature that lets you track offline sales triggered by online search ads.
The feature is called "Conversion Import" and it basically lets you associate an online search ad click ID with a sale placed in a physical store. All you have to do is associate the ID with the offline transaction. Google needs you to upload a file with that information and Google will then show you the reporting within the AdWords interface.
Here is a graphic of how it works:
Here is a video of Google explaining it in three minutes:
Here is a help document on the topic with a ton of technical details.
Forum discussion at Google AdWords Help.