That's what Heinrich Bullinger asserted in the Second Helvetic Confession. And he's not alone. Check out Luther:
"Tis a right excellent thing, that every honest pastor's and preacher's mouth is Christ's mouth, and his word and forgiveness is Christ's word and forgiveness... For the office is not the pastor's or preacher's but God's; and the Word which he preacheth is likewise not the pastor's and preacher's but God's." (Quoted from CD I/1, p107)
"When a man has climbed up into the pulpit... it is [so] that God may speak to us by the mouth of a man." (Sermon XXII on 1 Tim 3:2 "apt to teach", quoted in THL Parker, Calvin's Preaching, Westminster/ John Knox, 1992, p24)
Or, more to the point, check out the Bible!
"And we also thank God constantly for this, that when you received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers." (1 Thes 2:13)
For you have been born again, not of perishable seed, but of imperishable, through the living and enduring word of God. For, "All men are like grass, and all their glory is like the flowers of the field; the grass withers and the flowers fall, but the word of the Lord stands for ever." And this is the word that was evangelized to you. (1 Pet 1:23-25)
Remember your leaders, who spoke the word of God to you. (Heb 13:7)
So do we agree that 'Preaching of the Word of God is the Word of God'? Or would we rather Bullinger had maintained a more modest: 'Preaching of the Word of God explains and applies the Word of God'? Can we seriously maintain the word 'is' in that statement?
Karl Barth did. Emphatically. If you want to read more, go here to a very lengthy essay on Barth and preaching. Here I'll sketch out the argument in point form:
1) The Word of God is a three-fold Word. That is, Christ, the Bible and preaching are all called 'the Word' in the Bible. And yet there are not three competing words or revelations but One Word of God (Christ) who comes to us in the Spirit-mediated modes of Scripture and proclamation. Thus we have one Word in three modes. This is Barth's primary analogy of the trinity.
2) Just as in the trinity we have distinct Persons who, nonetheless, are one, so with the Word we have distinct modes which nonetheless have a perichoretic unity. The Son is one with the Father in His mediation of the Father. He is no less God for being a witness of God. But He is also no less distinct from the Father in this oneness. In the same way preaching is no less the Word for being a witness (a Scriptural witness) to Christ. But simultaneously it is no less distinct from Christ (and Scripture) for being one with it. We need a perichoretic ontology not only for God but for the Word also.
3) There is divinity and humanity to all three forms of the Word. Yet, for all that, we must avoid the danger of Nestorianiam - that is, we must not conceive of the humanity as a separate existence from the divinity. Barth is adamant that you cannot get around the worldliness of the Word - whether of Christ, Scripture or preaching. In fact, it is not at all desirable that you should get around it. For the Word as grace meets us where we are. Christ the Man says 'If you've seen me you've seen the Father.' Christ the Man says 'Son, your sins are forgiven.' The humanity of Christ in no way jeopardizes divine revelation or salvation. Equally, the humanity of the apostles and prophets and the humanity of the preacher does not prevent the Word from being still a divine Word.
Just as the eternal Word did not come in a man but as a man, so on Sunday morning, God's Word does not come contained somewhere within the preaching but it comes as this human preacher in this situation witnesses to Christ.
4) We must remember the divine initiative in all this. It is not a question of 'Can we hear God's Word in the preacher?' Rather the question is: 'Is it Christ Himself who encounters us in the preacher?' It's not a case of pulling Christ down through correct exegesis. If we think like this we're basically falling for an ex opere operato of the pulpit. That is, we're imagining that our correct priestly exercises ensure a divine encounter. We must resist this - we must begin from above. Revelation is grace. It is Christ who chooses to condescend in Scripture and Proclamation (not we who bring Him down). But in this divine condescension it is Christ Himself who encounters us.
Let's take all these points together. Preaching is a mode of the Word of God. It is distinct from Scripture and Christ but inextricably linked to it. And in relation to Christ and Scripture - that is, as Christ is proclaimed Scripturally - it is itself the Word of God. Not a competing revelation to the Bible but rather a 'Word from Word' (parallel to Christ's divinity as 'God from God'). The humanity of the preacher is not a barrier to divine revelation but instead is the very worldiness in which the Word must meet us. Thus the congregation on a Sunday morning is not confronted with explanation and application of the Word. They are confronted with Christ Himself.
Think of a preacher who challenges the congregation to confess Christ as 'My Lord and My God.' (John 20:28) If the hearer does not trust Christ, is it only the preacher they've disobeyed? Have they not more fundamentally disobeyed Christ? Isn't it Christ Himself who confronts them in this preaching? It is a daunting prospect for preachers, but such is the humbling authority of 'the keys of the kingdom' (Matt 16:19; John 20:23).)
[Preaching is] "the speaking of God himself through the lips of the minister." (Karl Barth, Homiletics, Westminster/John Knox Press, 1991, p67.)
"...in what Church preaching says of God, God Himself speaks for Himself." (Barth, Karl. Church Dogmatics, vol. 1, part 2, trans. Geoffrey Bromiley, Edinburgh: T&T Clark, 1956, p800)
This post contains reworking from my comments at Faith and Theology