From the Wizards of the Coast press release by Chris Tulach:
Explore the Chaos Scar in D&D Encounters: Keep on the Borderlands
The next season of D&D Encounters begins on September 22 with a classic-feel campaign: the Keep on the Borderlands! The adventure, A Season of Serpents, highlights the new D&D Essentials books as the characters explore the dangerous lands of the Chaos Scar near Restwell Keep. A traditional D&D play experience coupled with an amazingly fun story arc written by Chris Sims, this season is sure to be an exciting return to the days of D&D yore. I've been barraged with questions about this season, and now’s the time to give answers!
How long is this season?
D&D Encounters: Keep on the Borderlands is broken up into five chapters of four sessions each, for a total of 20 sessions of D&D play. The season starts on September 22 and lasts through February 2, 2011. At the beginning of Chapter 3, characters will bump up to 2nd level. When Chapter 5 starts, characters will become 3rd level.
How will character creation work with D&D Encounters: Keep on the Borderlands?
Players can create characters using the D&D Fantasy Roleplaying Game boxed set (A.K.A., the "Red Box") or the Heroes of the Fallen Lands book. The second book will have more options for players, but both are viable. Both will be available prior to the first session of the season. Once Chapter 3 starts, players can use the Heroes of the Forgotten Kingdoms book to make a new character or update their existing character if they want.
We're proceeding with a limited set of rules for character creation to make it easier for new players to jump in, and we think that existing players will love the new character builds in the Essentials series of books. The slayer, knight, thief, mage, and warpriest are "classic" character builds that fit very well into the feel of the season. In addition, DMs know exactly what rules material will be brought to the table, which should help them provide a better play experience for their group.
Pre-generated characters will be provided if you don't want to "roll your own." All of these characters are also built with the new Essentials character options.
What kinds of rewards can we expect?
We'll be giving out two D&D Encounters Reward Cards this season (Heroic Aid and Enchanted Heirloom), earnable with Renown Points as with previous seasons. We’re going to be including more cards in each kit as a response to requests from organizers to accommodate more players. You’ll need to play a little longer to earn the first reward (it requires 20 Renown Points), but the length of the season should ensure that you have many opportunities to do so over the fall and winter.