If you pre-ordered the iPhone 4, it should already be on its way. Here's how to keep track of your iPhone 4 delivery (or any package) on your old iPhone. Ah, the joys of tracking packages.
The expensive way: Delivery Status touch- $5
The main page tracks your packages with a big fat countdown to show you how many days left until delivery. Colors correspond to the service—purple for FedEx, brown for UPS etc—which adds a nice visual touch. When you click on the location of your package, it'll display your current location against the package's location which can either brighten your day or crush it if your package is still in China. Delivery Status touch supports tracking services for UPS, USPS, FedEx, Amazon, Apple and a ton more shipping services (more than 25).
Aside from a few premium features (barcode scanning, better push notifications) that a premium priced app should have, it's by far the best delivery tracking app on the iPhone. But at $5 it's probably not worth it unless you're tracking multiple packages a month or just an all around tracking fiend.
The free ways: Package Tracker Lite - Free
It's free, so it's basic. The app can only track packages from UPS, USPS, FedEx, and DHL but it keeps them all in an easy to read list. You get to re-name the tracking number into something relevant like, 'iPhone 4' or 'iPhone sucks this is a Droid' so you can be as organized as you'd like. After you get your list up, the information is basically a re-packaged version of tracking info found on a website.
There are ads but hey it's a free app. If you only track packages once every new iPhone, this is probably the app to get. It's not as elegant as Delivery Status touch but it gets the job done for those not maniacal enough for a paid app.
FedEx - Free
It's a solid app but requires a FedEx account to use. That's terribly annoying if you're just going to track your iPhone 4. Obviously, it's only limited to FedEx pacakages which means you can't get your brown on. But if you're only just looking for an easy way to find your FedEx'd iPhone on the go, it's a decent option. A little cluttered for pure tracking purposes but for those deeply invested in FedEx, not a bad app.
Safari (Order Status)
It's a little surprising that the Apple Store App doesn't have the ability to track your order status. You'll have to fall back onto Safari for this one but at least it'll keep your homescreen free from clutter. If that's your thing.