Update [2006-1-13 14:18:8 by AlSpangler]: Additional games are on April 11, May 2, May 18, June 15, July 20, August 3, August 24, September 7 and September 14 (Tuesdays and Thursday only). The Cubs' only Monday day games, Memorial Day & Labor Day, are not included.
Nine additional games, on Monday, Tuesday, and Thursday afternoons, will be offered as an option to those holding Double Play Season Tickets. You will need to take all nine games or pass on the offer.
If you have more than one seat, you will be allowed to purchase the extra games for only some of your seats.
I found this out while speaking with a Cubs ticket rep after someone holding the night/weekend season ticket package for my seats upgraded to a full-season package. I was bumped but actually ended up with better seats.
There are only 15 games in the 2006 Double Play package because there are only 3 Wednesday day games. Usually the Double Play Package had 21 to 24 games per season.