I would be inlclined to look at what has the best support on all platforms that you are going to use, I suspect the iPhone app may end up driving your choices.
If it was pure .net I would still tend to lean toweards a SOAP service - it is not considered cool these days but it generally will do what you need on most platforms without having to roll custom solutions.
EDIT
ASP.NET Web API means that .NET now provides a great framework for developing a restful API, I revise my answer to say that I would now lean towards this - progress is great!
