x2, it's probably cpm stuff, which shows up more when your traffic isn't worth much. do your stats show any placement targeted ads on your site? are you seeing a lot of adsense banner ads?
I have been going throug this issue of getting genuine clicks and no earnings. Wrote to Google Adsense customer support not to get any clear answer (they answered twice though, after 20 days). Finally, I reverted to Bidvertiser which is not earning as much as AdSense used to do but not bad. Ajith