What is Biblical Theology?  Since the question comes up frequently, I thought I should try to address it: 

First of all, Biblical Theology is more about a methodology than a result.  Biblical Theologians apply a certain set of exegetical, archaeological, literary, and historical tools towards understanding and interpreting the Scriptures as a progressive history of redemption.

For example, the Flood Narrative beginning in Genesis 6 introduces the concept of Baptism.  As the story of redemption unfolds, "trial by water ordeal" reoccurs with the story of baby Moses, the crossing of Israel through the Red Sea, and Joshua leading Israel through the Jordan into Promised Land.  In the New Testament, Baptism appears in the Gospels, Acts, and Pauline Epistles.  Several of the Writings also touch on Baptism, such as Psalm 69.

Subsequent "water ordeal" data points after the Flood build on one another and help us develop a rich concept towards understanding the meaning of Baptism.  Furthermore, the idea of "Baptism" or "trial by water ordeal" is further explained by archaeological evidence and various ancient near eastern texts.

After doing all this work, we come to appreciate that the "Baptism" stretches far beyond simple water immersion.  Baptism touches the recurrent Biblical themes of Judgment, Salvation, Vindication, Membership (in the Holy Community), and Loyalty, to name a few.

Consequently, this Historical-Redemptive approach often ends up with a range of results, rather than a single result, when applied to a concept or theme, such as Baptism.

The field of Biblical Theology is still relatively "new" in the sense that much work remains to be done in understanding and analyzing the numerous "threads" found throughout the Old and New Testaments.  Ideally, such studies are focused and exhaustive. 

Two excellent examples of Biblical Theology are Meredith Kline's God, Heaven, and Har Magedon and Timothy Laniak's Shepherds After My Own Heart, which examine the threads of the Mountain of God and the Good Shepherd respectively.

Wikipedia also has a reasonably good explanation of Biblical Theology.