CTR increased by 40% around Sept 4; earnings the highest so far since June - and we did not change a single thing in our site (no color change, no change in layout, no change in format). The only thing we are doing is to increase the content (articles averaging 600-2500 words) and putting Adsense leaderboard in the new content
CTR, EPC has remained fairly stable since the increase. No PSAs in my site, too - all ads are well targeted.
I don't think people are getting immune to text based ads. I am not seeing anything in my site to support this hypothesis.