I’m not sure what kind of file sharing you are looking for. If you need a cloud based file storage that supports syncing to devices such as iPhone, you can use SkyDrive, which is free. We have SkyDrive for iPhone, which can be found in App Store.
If you want to use Windows Azure, you can store the files in blob storage. But I am not familiar with whether currently there’re any iPhone clients that allow us to sync files between iPhone and Azure blob. However, you can also build
your own using iPhone SDK(microsoft-dpe/wa-toolkit-ios):