A WebmasterWorld post has a line of feedback from a Google representative reprinted in the forums that reads;
It is true that keywords which are inactive on search may still trigger ads in the content network.
Interesting. Now, the word inactive typically tells the advertiser that the keyword is no longer in the pool of ads running. But now it appears the ad may run in AdSense ads and not the sponsored search results in Google.com. Interesting...
Forum discussion at WebmasterWorld.