The first part of that question can be answered by what Mint team lead Clement Lefebvre calls the Mint Gnome Shell Extensions. The goal of the MGSE is to re-implement the known and proven GNOME 2.x desktop using GNOME 3 technologies. You can individually turn these extensions on and off; when they're all turned off, you have a vanilla GNOME 3 experience. Turn them all on, and you get a more traditional desktop.
"Gnome 3 is changing all that and is developing a better way for us to interact with our computer. From our point of view here at Linux Mint, we're not sure they’re right, and we're not sure they’re wrong either," Lefebvre writes, "What we're sure of, is that if people aren't given the choice they will be frustrated and our vision of an operating system is that your computer should work for you and make you feel comfortable. So with this in mind, Gnome 3 in Linux Mint 12 needs to let you interact with your computer in two different ways: the traditional way, and the new way, and it's up to you to decide which way you want to use."
At the same time, there are still people who want GNOME 2.x-proper. For these people, Linux Mint has worked very closely with the team behind MATE, the GNOME 2.x fork. The problem is GNOME 2.x can't be installed alongside GNOME 3 (at least, not easily); MATE, on the other hand, can be installed alongside GNOME 3. However, the project isn't there just yet, so Mint is working hard with them to get there.
For me, this is great news. I dislike both GNOME 3 and Unity, and I just want a GNOME 2-like experience - I don't have the time or patience to be a Canonical or GNOME guinea pig with their oversized buttons and lack of minimise. Linux Mint is starting to look like a good option here - and I'm not alone in that assessment, since Linux Mint is seeing extraordinary uptake, and may surpass Ubuntu this year.
Linux Mint 12 is planned for somewhere later this year - when it's done, basically.