The claim that "mobile providers reject all encrypted data" is not accurate. To my knowledge, mobile carriers don't block SSL connections or VPN connections. Perhaps there was a miscommunication or misunderstanding of what exactly Harald meant.
On the other hand, here's one thing that is true. Many cellular standards include some kind of link-layer encryption for the voice or data as it is transmitted over the wireless link between the phone and the base station. However, major flaws have been found in these encryption standards. Also, many mobile carriers will allow governments to tap into this data at the central office, after they have decrypted it. There have also been carriers who have turned off the link-layer encryption, and governments who have required carriers/equipment vendors to turn off the link-layer encryption locally (or when exporting cellular equipment).
But, if you have a smartphone, you can still use your own end-to-end encryption to protect your own connections -- mobile carriers don't block this. And if you use your own end-to-end encryption, or if you use apps that implement their own end-to-end encryption, those link-layers weaknesses and backdoors don't damage your security, because you've eliminated the dependence on the mobile carrier's link-layer crypto.
It is true that sending data over a mobile phone is likely to be less secure than a wired connection. This is because most phone apps don't use end-to-end encryption, and most data is sent without any end-to-end encryption. (For instance, most websites use unencrypted HTTP connections, rather than end-to-end HTTPS encryption.) Therefore, most data sent from your phone is only as secure as the cellular link-layer encryption -- and that isn't terribly secure.