It very well could be fraud. I would look at your statistics tomorrow...if you see a term that normally has 50 impressions skyrocket to 1200 and its CTR go from 3% to 18% then I would report it.
If you have a high quality log analysis software package you should be able to determine if the clicks were fraudulent or not, just by seeing if they stay on the page. The more info you can gather the better when you make a claim that you had fraudulent clicks...If they come back and say that they did not determine that you did get fraudulent clicks and you can prove that you did, you can get a refund/credit.
This can also be a problem with content syndication. I had one client who insisted using content sites for more exposure until they got 1500 clicks in an hour (for a site that's use to 1500 in a week from OV) when a newsletter went out that included their paid links.
Overture is allowing content syndication of ads in pretty much any way their partners want them to use it, which has caused a lot of account problems for people.