A user can see their highlights on the web after logging in by going to their "your highlights" page. As far as I know, there isn't an API, nor is there any sort of OAUTH thing that you could do so currently the best possible solution would be to get your user's amazon username/password and scrape them from that page.
It's better than nothing, but a real API would open up a lot of cool ideas that I've had on kindle highlights.
