Is it permissible to offer free IOS apps backed by paid subscription services?

So you want to use Apple's tools, platforms, customer base, devices, and infrastructure for free but don't feel that Apple deserves a cut of your ad revenue?

I must apologize for my reply. Somehow I read that you were looking to use a 3rd party advertising platform. I don't know where that came from.

You can certainly connect to a non-Apple, paid service. What trips people up is that you can't have a link to sign up for said paid service. If users are able to sign up directly in the app, they must do it through the App Store. It is a very good idea to have a native login to said service as well. If you use WebKit then the website could display a "sign up now" button that would get your app rejected.

All of the details are in the guidelines and developer agreement. You need a developer account to see them but people have posted then all over the Internet. In the developers site, they are listed right up at the top.