While you're right that all CAPTCHAs are annoying, the CAPTCHA ads are a whole different level of annoying. I think you'd lose more goodwill than you'd gain in money from doing this.
depends, I believe it will depend on what the user has to type.
I disagree. I'd find an ad in my CAPTCHA to be sleazy regardless of what I had to type. It's a bad idea, full stop.