Is God's forgiveness conditional?

In Matthews 6:14-15, Jesus proclaims to his gathered disciples: "...If ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses" (KJV). Does this statement contradict the key teaching of Trinitarian, incarnational theology that God, in Christ, has forgiven all people?

It's important to understand the context here. Jesus' statement is part of his Sermon on the Mount, (which spans Matthew chapters 5-7). In this sermon, Jesus is telling his disciples about life in his kingdom (life with him under God's rule). This context is helpfully addressed in studies on the Sermon on the Mount on the Trinity Study Center website (produced by Dr. Gary and Cathy Deddo):
Jesus begins [the Sermon on the Mount]...with a list [called The Beatitudes]. It is almost like a poem. Each of the first nine lines begins with the word “blessed.” Each of the first 8 lines has a second half that begins “for theirs is” or “for they shall,” thus giving a reason for why these people are blessed. If you study the first 8 lines, you notice that the first and the eighth end exactly the same: “for theirs is the kingdom of heaven.” This is present tense, meaning that right now, these people have the kingdom of heaven. In all the lines in between these two, the second half begins “for they shall...” These are all future tense, meaning that they will be fully realized only in the future. What is the significance of this structure? Well, I think it means a couple of things. First of all Jesus is speaking about the kingdom as a present and future reality. We can truly possess the kingdom of heaven here and now on the earth. We can enjoy the truth of this, at least, to some extent. But the greatest fulfillment of having this kingdom will, for us, come in the future. It is a real hope that affects our lives profoundly now, but one we look forward to seeing completely fulfilled. [Emphasis added]
Because Jesus has come to dwell with humanity, the kingdom is a present reality on earth. What Jesus describes is how this reality may be experienced by his followers. Though their experience will be only partial now (due to human weakness), it will come to fullness in the age to come.

The Deddos note that in Mat 6:14-15, Jesus is addressing forgiveness while making a point about experiencing the kingdom in prayer. A superficial reading of Jesus' statement might lead one to conclude that what we do (or do not do) conditions the Father’s desire to forgive us. However, what we learn when we consider the full scope of the Sermon on the Mount (and as we examine the scope of Jesus' entire ministry) is that the Father's forgiveness toward us is NOT conditioned upon what we do or do not do.

Jesus' statement warns his followers not to succumb to the temptation to resist God's unconditional forgiveness, thus failing to experience in their lives its full benefit. Jesus' point seems to be that if we, as Jesus' followers, cling to a heart of unforgiveness (harboring ill will toward others) we will be unable to receive the forgiveness that God, in Christ, has extended to us already.

In short, hearts closed toward our neighbor are also closed toward God.

However (and this is a vital point derived from the whole of Scripture), our unwillingness (or inability) to forgive others has no power to change the Father's mind about us. His mind toward us is conditioned only by the finished work of his Son, Jesus Christ, on our behalf. In Christ, God has made up his mind once and for all about us. He has forgiven us all.

Some will object, saying we should take what Jesus says "literally." After all, the statement reads like a cause and effect (tit for tat) equation: “If you do not forgive.... neither will your heavenly Father forgive you.”  However (as noted by the Deddos), the grammatical construction of the statement (given as a warning) and the context of the whole Sermon and of Jesus' entire ministry, indicate that what Jesus is addressing here is not an action that God takes toward us, but the result in our own hearts of actions that we take (or fail to take). Note the following comment from the Deddos:
The purpose of this warning is to...prevent someone from not being/receiving forgiveness... Giving consideration to the larger biblical context as related to God’s forgiveness, whenever Scripture speaks from God’s side, and tells us directly of God’s character and purpose we see that God is not conditioned by us, and is of one mind and character towards us. He is gracious, forgiving, redeeming. It is clearly his purpose to save, not condemn.
So even though the result in this warning is expressed in the indicative, since it is set in the context of a warning, there is no grammatical reason to take it as purposive, revealing the aim and intention of God. The consequence should only serve to indicate a result and one that is contrary to God’s purposes. It should not initiate a reinterpretation of the character of God. Note that even here God is addressed as our heavenly Father. Whether we receive or somehow manage to refuse his forgiveness, God remains our heavenly Father because Jesus remains our heavenly Brother.