I came across this quote attributed to Rowan Williams, ABC, and wanted to use it for the Imago Dei and Red Hook efforts.
I googled the quote to try to find the original source for a citation. It seems from an address given by The Rt. Revd. Dr. Steven J.L. Croft, Bishop of Sheffield, that Williams was not the originator of the quote. So, in the larger context of the address, Croft was speaking of missionary theologians who spent most all of their ministries in mission contexts and who later reflected on their experiences and wrote down those reflections. Croft mentions a few that to him have had a great impact on Anglican mission ideas. It seems the quote came from , The Rev. John Taylor. Here is the quote in its small context:

John Taylor was a CMS missionary to Africa and former head of the Church Missionary Society. His books, particularly the Go Between God are a clear articulation of a theology for mission which have influenced a generation of theologicans, bishops and practitioners in the United Kingdom. It is John Taylor who first articulated the wonderful phrase which is now widely quoted in Anglican documents on mission:
“Mission is about finding out what God is doing and joining in”.
One of the key elements in the cluster of ideas around missio Dei is that the Trinity is already at work in the world outside the church. God continues to reach out, to call, to love those he has created. We therefore go, as the Apostles went to Philippi so long ago, confident that we will find those who are seeking.
I hope I have said enough in this section to convince you that the movement to develop fresh expressions of church by the Church of England has deep roots both in the recovery of a sense of the missio Dei in world theology; in biblical studies and in our recent practice and exploration. It is the logical and practical outworking of shifts in our theological perspective which go back now a generation. It is not the mindless pursuit of the trendy or of consumer Christianity.

The entire address of Bishop Croft can be found in his address to the General Synod of the Church in Norway; 18th November, 2009; entitled:
The Mission of the Triune God Shaping Congregations Today Working towards a Mission-shaped Church
The Rt. Revd. Dr. Steven J.L. Croft, Bishop of Sheffield
Here is the HTML version (don’t know if it will work in all browsers):
http://tinyurl.com/yktjurd
The PDF for download came be found here: http://www.kirken.no/?event=downloadFile&FamID=102589