About a week ago, many Google Analytics users and Google AdWords advertisers began noticing that the AdWords cost data was no longer being sent to the Google Analytics reports. We said that Google sent a warning that this may happen, if you don't link the two accounts.
Googler, AnalyticsPro.Chris, created a new "sticky" thread at Google Analytics Help discussions that talks about this issue.
Going forward, you'll have to tag your URLs and enable cost data to see AdWords data in your Analytics account.
If your accounts stopped receiving AdWords CPC/PPC data around March 23/24, It is possible that it is affected by the cost data changes implemented then. If you are an admin on such an account, you should have received a message, in February, with the subject line: Action required: Important change to AdWords/Analytics cost data Here's an article explaining the different scenarios: http://www.google.com/support/analytics/bin/answer.py?hl=en&answer=57167
So if you have this issue, hopefully this will help you going forward.
Forum discussion at Google Analytics Help.