The unity iPhone scripting reference (Applications/Unity iPhone/Documentation/ScriptReference) just seems to list all of the unity classes with no indication of what has been implemented on the iPhone.. eg: the Network classes are listed, but as far as i know are not implemented. How do you tell what has been implemented for the iPhone?
We could definitely recommend our documentation in this area. Barring that, what I often like to do in this case, is (on windows), download Reflector, and use it to look inside UnityEngine.dll. that should give a pretty good overview of what's available.
You can see what is and is not available on the iPhone by taking a look at the license comparison page. Although it doesn't list specific classes, you should be able to infer which classes are available and which are not based on what is supported and what isn't. Also, some features (such as .NET sockets, and therefore the networking classes) are available only with the iPhone Pro license, not the iPhone Basic license.