Credit goes to BCBer Doggie Stalker who broke the news in the pregame thread.  Here is the memo (for combination plan season ticket holders):

Note: This is the earliest in the season that I have received the postseason invoice (this is about the 6th or 7th year I have received one).

Dear <Awesome Cubs Fan>,
It is my pleasure to inform you Major League Baseball has authorized us to prepare for post season play. The enclosed fact sheet indicates the post season schedule and pricing structure. The ultimate schedule is determined based on winning percentage and which teams qualify as division champions and “wild cards”. It is possible we will not know which games will be played at Wrigley Field until the very end of the 2008 regular season.

We are only able to offer you a slight increase in the amount of tickets that you are entitled to through your combination season ticket plan. We are offering you the ability to purchase the same number of tickets as listed in your account for all three games in the Division Series, two randomly selected games in the League Championship Series and two randomly selected games in the World Series.

Payments for postseason tickets are due in full by September 8th and may be paid by personal/company check, money order, MasterCard, Visa, Discover or American Express. For payments received after this date, we cannot guarantee seat location or tickets to post season play. If the Cubs are eliminated from post season contention before playoff tickets are mailed, we will automatically credit your payment to your 2009 season ticket account unless you have indicated on your invoice that you wish to receive a refund check.

Thank you for your support of the Chicago Cubs as we look forward to playoff baseball again in 2008.

If you have any questions, please call us at <season ticket phone number>.

Sincerely,
Frank Maloney
Director of Ticket Operations

Edit: Playoff Schedule and pricing shown below