Would it be against the TOS to create a post in your blog that said something like: "New adverts have been placed to support the site. The money made will go towards competitions, site improvements and more" If not (which I suspect), how about just a post that just says "I'm afraid I've had to put up some adverts" I'm tempted to put adsense on a blog that's currently nonprofit but feel I will need to justify it to my user base to prevent them from becoming annoyed. Thoughts?
Definitely against TOS, you can't attract attention to the ads. Just put them on and continue your life.