Am trying to find a Content Management System that I can use to produce and store content - text, images, PDFs, etc and then access this content from an iPhone using a native application via a web service.
Can anyone recommend a good CMS for this?
You can take a look at StorageRoom, which is a flexible cloud-based CMS for Mobile Applications.
Disclaimer: I was in the same situation as you are, and didn't find anything myself, so I created this myself (I am the founder).