Historically iPads and iPhones have not allowed you to write comments and forum posts on TPN. I upgraded my iPad and iPhone to iOS 5 and it now seems to work properly with TPN. I'm posting this message from my iPad which I couldn't have done prior to the iOS 5 upgrade.
I tried posting a picture. That doesn't work, but at least you can view and comment on images and participate in the forums.
If you are an Apple mobile device user, I encourage you to upgrade if you haven't already. There are many excellent benefits and features in iOS 5 that make it worth the effort.