A WebmasterWorld thread discusses a possible bug many Google AdWords advertisers are noticing. Basically, it appears that many of these advertisers are not getting any traffic from Google's content network, even though they have their content network settings configured to retrieve traffic.
The bug started on around July 1st, and Google is apparently aware of the issue. They replied to one advertiser saying:
Thank you for your phone call today regarding your ads ceasing to show on the content network. I'm writing to let you know that I have escalated this issue to our technical team. They're currently investigating the situation, and we'll contact you as soon as we've found a resolution. We thank you for your patience and apologize for any inconvenience.
I hope this is fixed soon, I suspect the AdWords office was closed for the July 4th holiday.
Forum discussion at WebmasterWorld.