The sale will be conducted at the Apple retails stores as well as the online Apple Store.  Apple has posted a teaser at the online store:

"Come back to the Apple Online Store the day after Thanksgiving for a special one-day-only holiday shopping event. You'll find dozens of great iPod, iPhone, and Mac gift ideas — all with free shipping.

Mark your calendars now. And until then, start your research by visiting our Holiday Gift Guide to find iPod, iPhone, and Mac gifts for everyone on your list."

In prior Black Friday events (2006 sale details), Apple has offered discounts on select Macintosh computers and accessories.  Apple will announce the items and their sale prices on Friday.

In related news, Apple has posted their special holiday hours for the company's retail locations.  Most of Apple's US retail stores will be closed on Thanksgiving Day (November 22) and Christmas Day (December 25).  On December 31, most stores will close earlier than normal.  Sale prices will be limited to stock on hand and while supplies last.