What is the significance of Jesus' baptism for a theology of baptism? It seems that any robust theology of Christian baptism must incorporate and draw from the fact that Jesus himself was baptized, but I rarely hear it mentioned in discussions about baptism. What place does it have in a theology of baptism?
There is an excellent discussion of just this topic among the first parts of St. Nicholas Cabasilas' Our Life in Christ. Any discussion of Epiphany/Theophany also should address specifically this question.

Some thoughts along this line include the following:

In becoming baptized, as the other answers mention, Christ shows us a critical step on the way to salvation, viz. baptism by water and the Holy Spirit. In becoming baptized, Christ's holy body sanctified the water that now baptizes us into his death and resurrection.

Also, in baptism, Christ's divinity was made publicly manifest, by the full Godhead no less, and as a show against the prince of this world that the kingdom of God was now coming in subtle force. To further indicate how fully the kingdom was coming, Christ went out into the wilderness to meet Satan head-on. Now when we become baptized, we too spit on the works of Satan, and now on God's side we too meet Satan to drive him out, not to tolerate his works anymore in these last days.

This by no means exhausts the relationship between our baptism and Christ's. The above references should discuss this at length.