BillyS - 9:43 pm on Jan 15, 2011 (gmt 0)
If you're in the double digit zone, you might worry. A CTR that high is usually a result of either poor content or ads masked as content.
This last statement assumes you have a decent amount of traffic (>5,000 impressions per day). Anything less than that will have numbers that jump around quite a bit.
Finally, if you're not encouraging clicks, then you have nothing to worry about.