This is going to be long so take this as a fair warning.
As an Eagle Scout, long time Troop Leader, former Cubmaster and father of a Girl Scout and Boy Scout, this is my take on this. The addition of girls to the Cub Scout and Boy Scout programs is about declining enrollment and money for the BSA. I think that a lot of the declining enrollment is due to year-round sports, baseball and soccer being the biggest culprits of this. This wasn't an issue when I was a Scout 35+ years ago. Another thing driving it in my opinion, is that the National Office of BSA is deathly afraid of the Mormon Church pulling out of Scouting. It has been their youth program for boys for over 100 years. A lot of this is about money. Anther thing to remember is that the Chartering Organization (church, American Legion Post, VFW, etc.) own the unit in Scouting. If the Chartering Organization decides not to allow girls or gays, that is their prerogative and the BSA says that they will support them.
Now to program specifics, Cub Scouts (Kindergarten through 5th Grade) will have co-ed Packs, but gender specific Dens. This program is school grade specific so you will have a Tiger Den (1st Grade) for boys and a Tiger Den for girls. This will be consistent for every grade level in Cub Scouts (Kindergarten through 5th Grade). Since it's a problem to get leaders for Dens, a suggestion that I made was that when someone comes to register their daughter in the Cub Scout Pack, welcome them and when you give them an application for their daughter, you also give them an Adult Application and let them know that if they want their daughter in Cub Scouts that they are expected to register and be active as a leader in her Den. The Chartered Organization for the Pack that I was Cubmaster of has decided to allow girls in the Cub Scout Pack starting in August. We'll have to see how it works.
As for Boy Scouts, now Scouting BSA (ages 11 to 17), there will be gender specific Troops if a Chartered Organization decides to allow girls into the BSA program for this age group. The Chartered Organization for my son's Troop has decided not to start another Scout Troop for girls, leadership and equipment being the main reasons along with getting adult leaders.
On a personal note, my 10th grade daughter who is working towards her Girl Scout Gold Award (equivalent to an Eagle Scout) would really like to join the BSA program and earn her Eagle Scout but her age won't allow her to finish both. She says that the BSA program is much better than the GSA program and I agree with her. Personally, I think that girls should stay in Girl Scouts and boys in Boy Scouts but I understand where BSA is coming from and don't have that big a problem with it.