I don't believe you often see Google AdWords representatives talking about click-through rates. But in a Google AdWords Help thread, I spotted AdWordsPro, an official Google representative, taling about averages.
In short, he said that a good average to aim for in an ads click through rate (CTR) would be 2%. But he/she has seen CTRs in the double digits for campaigns that are constantly tuned and tweaked by advertisers. Let me quote AdWordsPro:
On the other hand, to give you a point of reference to shoot for, an average CTR is probably in the neighborhood of 2% - meaning that for every 100 impressions, you would have received 2 clicks. I personally don't think that a CTR statistic is particularly meaningful or even valid until about 1000 impressions have been received, however.
Advertisers who have really taken the time to learn how AdWords works, and who spend lots of time managing their account and tuning things up often 'earn' a CTR in the double digits.
Then, once you become a little more comfortable with AdWords, then it is time to stop focusing mostly on CTR, and instead focus on ROI or your Return On Investment.
I have heard of incredibly high CTRs, especially for Google ads with thongs in them. But outside of that, you know what to generally aim for.
Forum discussion at Google AdWords Help.