Apple is about to introduce an iPhone trade-in program that will allow users to walk into a Retail store and trade up from an old model to a new one. We’ve heard some interesting details about the way that the program will work, and indeed, is working right now in some pilot Apple Stores.

The program, has gotten a bit of press today, with Macrumors‘ Eric Slivka reporting that the training for the program is underway and Mark Gurman of 9to5Mac quoting a start date sometime in September.

But the trade-in program is actually already being piloted in some Apple stores, we’ve discovered. Those stores aren’t just preparing internally for an eventual program, they’ve been offering trade-ins to customers for several weeks. According to a source, this program was presented to them as something that may not be available at every Retail Store.  Some of these pilot stores have been processing ‘multiple’ trade-ins a day at this point.

The program works like this. A customer brings a working, non-liquid-damaged iPhone into an Apple Retail Store. It’s then evaluated by an employee with the help of an online site that a customer can access using the store’s iMacs or iPads. The customer then answers a series of questions about the condition of the device in order to determine a value.

This procedure is similar to the way that Apple handles its iPhone recycling program now, but that is by mail only, covers a wide variety of products and is not offered in-store.

Values can range depending on a variety of factors, including device color, physical damage and liquid damage. Though the prices could very well be tweaked before the program goes wide, the range is said to be around $120-200 for 16GB iPhone 4 and 4S models.

That value can be used in credit for a new device but only if the customer has an upgrade credit available. So there is a carrier check involved. If a user does not have an upgrade credit, they could presumably pay the early termination fee of their carrier and use the credit towards a new device on another carrier.

Currently, the devices are dropped into a bag and presumably shipped off somewhere for refurbishment and resale. They are not resold at the store where the trade-ins are being offered. So far, customers have been pretty excited that this option is now available at Apple Retail stores.

Obviously, this is how the pilot program is working, there could be some details that change between now and when it’s set to go live. We’ll let you know if anything material changes. We’ve reached out to Apple for comment on the new program, but have not heard back.