Ask HN: Contextual ads for web-apps like todo-lists or mail?

I'm creating a webapp that will deal with user-generated data in a one-to-one interaction behind a login and a password. I'm creating something that can go into a niche of todo-lists.

I don't think charging people is a good idea but ads should be pretty non-intrusive.

Is this ethical (granted I explicitly say what I do)? From what I know, GMail parses my private data for ad purposes.

Can the "default ad-network" -- AdSense do this automatically? From what I understand, it crawls webpages very very rarely and even authorizing it to access protected pages isn't going to speed up the process.

Is there any other solution for fast, document-based advertisiment?