What a great topic for thought!  Could probably write a book. . .I haven’t thought it out thoroughly, but here are some initial thoughts, probably poorly articulated:
 
1) 1 Corinthians 12 outlines the function of the believers as a body.  I believe the church is to function as the physical manifestation of the body of Christ; a framework for believers to join with others in exercising their gifts and callings for the common good of the body.  
 
2) Acts 2:42-47 was the first passage to pop into my mind as I read your questions.  It is a passage which outlines the example set by the apostolic church:
      42They devoted themselves to the apostles' teaching and to the fellowship, to the  breaking of bread and to prayer. 43Everyone was filled with awe, and many wonders and miraculous signs were done by the apostles. 44All the believers were together and had everything in common. 45Selling their possessions and goods, they gave to anyone as he had need. 46Every day they continued to meet together in the temple courts. They broke bread in their homes and ate together with glad and sincere hearts, 47praising God and enjoying the favor of all the people. And the Lord added to their number daily those who were being saved.
I see in this passage evidence of teaching, prayer, fellowship, discipleship/evangelism, mercy/outreach, and corporate praise/worship.
 
3) 1 Peter 2:9 would be another passage I would consider when answering these questions.  There are two initial points I draw from it:
      a) a royal priesthood:  the concept of the priesthood of believers is one that should not be overlooked when considering the purpose of the church.  As a framework for the functions of the body, the church must allow for those activities which enable believers to fulfill their responsibilities of their God given priesthood. . .prayer, evangelism, intercession, etc
 
      b) a chosen people/ a holy nation/ a people of God’s own possession:  I think we too often think of ourselves as Christians in New Testament terms only.  But Christ was the fulfillment of God’s covenant with Abraham, and the reference here to being a chosen people is a reference to the fact that all believers are considered children of Abraham and are therefore heirs to the covenant (2 Co 11:22, Ro 4:16, Ro 9:8 ).  Therefore, I think any consideration of the purpose of the church should include a study of the traditions of OT Israel, particularly in regard to corporate worship.  While some of the OT commands were made obsolete with Christ’s atonement, such as sacrifices, there are other things which reveal to us things about God’s character, which never changes, and therefore should be considered still relevant.  One example would be the use of music.  Read through Ezra and Nehemiah.  God hand picked musicians and singers and appointed them to return with the builders to Jerusalem.  God made music a priority. I think that reflects something of His character. God is the same yesterday, now and forever.  I would assume that music should still be a part of corporate worship.
 
Oh. . .how I could ramble on.  But I will do everyone a favor and log off. . .besides, it’s almost time for Boston Legal.  Priorities, you know.  

Todd,
 
Great topic for discussion.  Good thoughts already from Willa.
 
My initial thought is that the question might not be "what is THE purpose" as much as "what are the PURPOSES" (plural) of the church.  
 
One church mission statement (intended to be a one-sentce summary that basically answers your question) that I liked was "To know Jesus Christ and to make him known."  But even that one sentence squeezes in two concepts - both of which need quite a bit of fleshing out.
 
I think Dallas Willard touches some on your question in "The Divine Conspiracy," although that book is not primarily about the local church body per se.
 
I think John Stott has written some good stuff on this as well.  I'll see if I can dig that up.
 
Some purposes/activities I can think of:
 
Evangelism - bringing people into a living, life-changing personal relationship with Jesus Christ.
 
Service - to the poor, disenfranchised, widow, orphan, less fortunate, etc. First in the local body, but also in the community and larger world.
 
Discipleship/teaching/edification - moving people who have been evangelized deeper into knowledge of and relationship with Jesus Christ and exploring the implications of faith in Jesus for all of life.
 
Worship/praise/adoration of God.
 
Maybe you could say that "to make disciples" is the mission or purpose of the church.  But then of course you have to go on and define what it means to be, or to make, a disciple.  You have to say that by definition a disciple is (a) one who has been evangelized and (b) who is serving others and (c) is growing in knowledge of and relationship with Christ, and who (d) worships God through these and other means including the weekly gathering with others, etc.
 
Any answer probably needs to take into account the two great commandments - love of God and love of neighbor - and their implications.
 
Good food for thought.