You can either just buy the microcell and then it will still use your normal monthly minutes, or you can buy the microcell and get an unlimited monthly plan that would allow you to get unlimited minutes when connected to the microcell. This would work out well for people who work from home, or if someone has a home phone, they could get rid of it and only pay $20 for unlimited minutes through their microcell. It also seems that you get a $100 MIR if you sign up for the plan, making the microcell only $50.
Went to my local Saint Louis (south county) ATT store today. Although I kind of already knew the answer, I asked them if they knew what the 3G Microcell was. They had no clue what I was talking about. Looks like Saint Louis is not an early launch city. :-(