I’m interested in developing an iPhone application using Visual Studio and C#.NET on Windows Vista.
I tried to download the SDK from http://developer.apple.com/iphone
The iPhone does not support .NET. I don't believe there are any compatibility hacks or anything like that to run .NET apps on an iPhone, so you might have to bite the bullet and learn Objective-C.
I would do a google search for "iPhone development tutorials." There will probably be a bit of a learning curve.