Apple has released a new Apple Store app [iTunes link] into the App Store that allows iOS based devices to browse the Apple Store, check technical details and read reviews of products. If you're suitably convinced the Apple Store app will also show you where your nearest bricks & mortar Apple Store is, or allow you to purchase via your mobile device.
If you're more interested in a pre-release item, the app will also permit pre-ordering - that should be handy for high-interest products like the iPhone 4. Meanwhile support queries are also catered for, with the ability to make appointments for the Genius Bar, One-to-One and Personal Shopper services.
The app is a free download but currently only available for US users.