A well-known author on small group ministry—one you've probably read and heard at small group conferences—once told me he could not get past my father's statement that "the cell group IS the church, not a subset of it." (A statement made in Where Do We Go From Here?)
As we talked further, his major objection to the statement was that by taking it at face value, folks might get the opinion that the cell group or holistic small group was an independent, autonomous house church launched by a traditional church with a brick and mortar facility.
Even after explaining to him that my dad's comment was surrounding the fact that the presence of Christ in the midst of a small group of people creates a working definition for the NT church, he still couldn't move beyond his traditional paradigm to see what my dad was saying.
This is an important distinction. If one sees their small groups as one ministry among many, or sees them as supportive of another part of the church, they will live up to that expectation and never be transformational for the members or the community around the homes where the small groups meet.
Healthy small groups understand they have two functions, a mission and a ministry. Rick Warren said this in the 40 days of purpose video toward the end of the series. He went on to say that the ministry is to one another and serving in some capacity on weekends for the corporate gathering. The mission was to reach out as a group to those who did not yet know Jesus as Lord and Savior.
I find what Warren said to be spot-on... when a small group is actively pursuing a ministry and their mission, they're the church.
Conversely, I do not see many small groups in America doing this, even the groups at Rick's church (ok, there's probably a hundred or more groups at Saddleback that could be pointed out as doing both M's wonderfully, but among thousands of groups, it's a very small percentage). Knowing what to do and doing it are two very different things. I believe the success of a group doing both M's has a lot to do with how the groups are viewed by the senior pastor and the church leadership and most importantly, how they are launched and populated.
Populate groups through your Sunday services primarily and you'll always struggle to get them to do anything for God.
Neil Cole hit the nail on the head with this quote: "What you call them with is what you call them to."