Stores like Circuit City (hmm, maybe not the best example) have had programs allowing you to order an item online, then pick it up in store. For a company known for being on the cusp of trends, Apple hasn't entered the in-store pickup game, until now. And even now, it's quite a tentative step.
From Dec. 15th to the 24th, you can reserve a product from the comfort of your home, then pick it up at your local Apple Store. Beyond the limited dates, there are a few other caveats, namely that only a select group of merchandise is eligible for the program. Specifically, computers and iPods are available, while third party items, and Apple's own magic mouse, AppleTV etc are missing from the list.
Traditionally, the policy at most Apple stores has been to hold an item if someone called for it, for up to 24 hours. No word on whether that will continue while this program is in place. Also unlike the Circuit City pickup concept (again, Apple probably doesn't want to copy what they did too closely) you do not pay for the Apple items until you pick them up.
This isn't a bad addition for the customer, but I feel bad for my former Apple retail compadres who will have to deal with a sea of back of house, waiting for pickup, merchandise. According to the reservation system, you can pick up your item(s) anytime in the Dec. 15-24th window, meaning the item won't be unheld until the 25th. You don't want dozens of iPods sitting on the shelf, never getting picked up if people actually in the store want to buy them. Heck, Apple should watch out for millions of iMacs being reserved by say, a certaiin Redmond company that then doesn't pick them up...
Also, while it's clearly designed to help cut down on the lines and waiting at the store, it seems like a dangerous time to roll out such an initiative. A slow October roll out may have given them a chance to work out the kinks before the holiday onslaught.
If you know what you want, and it's on the list of eligible items, then by all means give it a shot, and be sure to tell us about your experience.