What you're probably wowed by is Photoshop - one of the secrets to a really good looking iOS app is to create many of the assets in PhotoShop.

If you're wondering where to get the underlying controls - http://cocoacontrols.com/ - Cocoa Controls is a project of someone here on HN, I believe - I usually start browsing there first. You can usually find replacement classes - for instance, I used DCSwitch instead of the stock UISwitch for a client.

Also, with iOS 5, UIAppearance really helps a lot.