It is possible to write any type of app you want for iPhone on a Windows based computer if you have the right software. What will work out best for you will depend on what type of app and if you are using Flash or not. For example a great choice if you want to use Flash is Flash CS5.5 because it has a packager for iOS for creating the apps for iPhone or any Apple device.
Another software that is good for writing iPhone or other Apple devices on Windows is Marmalade. This is one of the more expensive options, yet if you want to build 2D or 3D games it is considered to be one of the favorites.
Other Good Choices to build iPhone Apps on Windows:
- Unity 3D
- Stonetrip S3D
- Appcelerator Titanium
- Genuitec MobiOne
- Dragonfire SDK