In what is one of the most moving and powerful expositions on Paul's letter to the Romans that I have read, The Bishop (N. T. Wright) intrinsically asserts the full nature of this statement to be bigger than typical renderings through his placement within the Pauline context. In other words, Paul is saying something which most modern Western ears are not hearing. While most people see this as a personal motivation for witness, the very Jewish language invokes a Jewish background to the (very Jewish) Messiah - whose kingdom Paul is summoned to be a slave (1:1). Wrights discussion (423-426) on Romans 1:16-17 explores the implication of 'shame' as it is brought into the boldness of Paul's declaration.
NTW: "'Shame' in such a context is what God's people feel when their enemies are triumphing; it is what Israel (and many other peoples) felt in Paul's day, suffering at the hands of Rome" (424).
A great example of this (which is also cited in Wright's commentary) are the psalms, which express the shame experienced by God's people on their subjugation to those who oppose God (cf. Psalms 71:1-2; 31:1-3; 143:1; etc). When this is coupled with the understanding of gospel which is rightly taken to mean the ascendency of an emperor to his throne (the 'good news' of his reign), we have a different statement altogether than what is typically preached from the pulpit on this verse.
In short, Paul's statement is about vindication. This can be seen not only in the reference to the Psalms, but also the Second Temple context which shaped and colored Paul's theology. Thus, this statement is about the resurrection (of Jesus the Messiah), ascension (of the good news), and the vindication of Jesus which makes both possible (not ashamed) - only, now it is the status of Jesus as a shared experience of all believers (I am). Rather than this being a simple statement of personal boldness, it is a declaration that his bold proclamation is built upon the work of God through Jesus.
"The explanation is that the gospel, this message about Jesus that he has outlined in vv. 3-4, is itself God's power. It not merely 'possesses' God's power or 'is accompanied by' God's power but simply is God's power" (423). And this is the passion and purpose of every believer, not placing the winning or losing of the impact of our faith upon our own shoulders but that we should do our work well and allow the success of the gospel to be God's responsibility.
Typically, Christians try either to do too much or too little of living out their faith. Too little in that they simply want to *hope* for a better world without personal commitment. Too much in that they make the Christian faith more about the individual believer, or the local church, or the church movement than about letting this story be about God's work through Jesus. And this is what Paul is getting at all along.
For I am not put to shame for the good news of the Lordship of Jesus the Messiah, because it is the power of God for salvation to all who believe, first for the Jew, then for the Gentile.