I've never really experimented much with my adsense placements. I'm an international publisher, and only recently were the firefox refferals enabled. Now, I run a website with downloads and on the main download page for each file (where users finally start downloading..) I've made a statement saying "Download problems ? Try using Firefox:".
This statement is completely true since most people on my site have trouble downloading with IE (we have ftp downloads and IE tends to mess up sometimes..).
Anyway, below the statement, there is a link to a PAGE on my website which explains the benefits of Firefox and has the Google Firefox link in it.
Now, I was quite sure up until now that this would be alright. However, when I made this change (added this thing to the download screen) my firefox earnings have jumped from ~$5/day to ~$35/day. Call it magic ad placement or whatever, but would Google consider this incentivising ?
I'm not stopping IE users from downloading or anything, just recommending that Firefox might help you if you're having download issues.