We have questioned if Google limits how much an AdSense publisher can earn in the past. We even ran a poll where the majority of publishers felt Google did not have a glass ceiling for AdSense earnings.
I 100% guarantee that there are no earnings caps on AdSense accounts. I will swear it on a big stack of Google search results. No earnings caps.
That is believable to me. Do you believe it?
Forum discussion at WebmasterWorld.