UPDATE: The problem has been resolved

Since January 9, Google Analytics has been experiencing technical problems capturing traffic data coming from Google AdWords cpc platform.

While there is no official word yet on the cause of the problem, we are in touch with Google Tech Support, and Google engineers are working on it. Our initial suspicion is that the situation is connected with Auto-Tagging, a setting that enables advertisers to pass campaign information into Analytics without manually tagging their links with tracking information.

The problem does not affect the customer’s experience. Customers coming from paid Google ads are still able to place orders, and if you’re using Google Adwords conversion tracking, orders are still being tracked as conversions within the AdWords platform.If your Google ads are manually tagged using Conversion Ruler, Atlas, Omniture or other third-party tracking tags, you’ll be unaffected.

The problem is apparently affecting a number of other sites. Here is a thread devoted to the topic at Google Groups:

We will keep you updated as soon as we have more information!