NASA has built a great website to see a listing of ISS sightings by city. It comes in tabular form with an accessible RSS feed. For example, here is the listing for Berlin, Germany.

Useful, but what I needed was a calender instantly accessible from my phone.

You can use the two below projects to create such an .ics file and host it somewhere, such as your public Dropbox folder. Then subscribe to that location using your calendar app.

I personally use Apple's Calendar apps on Mac OS X and iOS to test this. Google Calendar doesn't complain though. Your mileage may vary.

It's only 'developer friendly' right now, and a work in progress. It wouldn't take much work to create a user friendly front end.