I've been more delibrate lately looking at how some services I am familiar with deal with the app store. It seems there are many businesses that make the app 'free' in the app store but require a login once installed. One example is Agworld. The login is only available if they are subscribed through the website. Clearly Apple have been allowing this for a while. I guess you just can't have a link for payment in the free app but its ok to ask people to login with some external credentials?
