How to make a jailbroken iPhone appear un-jailbroken to network provider?

The only visible change that jailbreaking does is the installation of the cydia app on your homescreen. what i would suggest is to go into cydia and get an app called Poof to hide the cydia icon. You can follow the guide here http://www.iphoneappsandhacks.com/how-to-hide-icons-on-iphone-or-ipod-touch.html, or you can hide it in a folder with a bunch of other apps.

The network provider can't see if it's jailbroken or not. They might be able to detect if you're using Skype over 3G or other non-3G apps but they won't be able to discover it by simply jailbraking it.

If that's true why can some apps detect it anyway? I installed e.g. Voddler on my iPhone yesterday; a free app to see movies without a charge. I can't see any movies because the app detected my iPhone as jailbroken and I got a message saying that jailbroken devices are not allowed to play the movies.