I feel much more tentative in advancing a contradiction of this interpretation than I did regarding Acts 17, and I would be very interested in comments.
Firstly, there is revelation going on in this passage - "the wrath of God is being revealed from heaven". However, if we ask to whom this revelation is made, the answer seems to be that it is made to Christians. If we read verses 17 and 18 together, we get: "For in [the gospel] the righteousness of God is revealed from faith to faith... For the wrath of God is revealed..." In other words, from the standpoint of faith in the gospel it is possible to see the wrath of God revealed in the way pagan history has played out. The pagans themselves, of course, do not see this.
Secondly, this passage is a description of pagan history. In chapter 2, Paul will go on to discuss Gentile responsibility to God, and then move to the question of whether the Jews escape condemnation through having the law. So it seems most sensible to read this second half of Romans 1 as Paul's review of how the Gentiles got into this mess in the first place. I think this view helps to explain the phrase "ever since the creation of the world", and also helps to explain Paul's description of decline from a high level of knowledge. Obviously, at the beginning there was recollection of God's personal revelation - his close personal fellowship - with Adam and Eve. But this has been gradually squandered. Knowledge had been exchanged for ignorance. (Doubtless this does also describe the general trend in societies which neglect God, but I think Paul is here describing history, not sociology).
Thirdly, this perspective on pagan history can only be delivered from the point of view of Biblical faith. I am uncomfortable in the extreme with the use of this passage to say "everyone does actually know that God exists, even if they won't admit it - they're just suppressing the truth". Paul's description of history points to the conclusion that people actually do not know that God exists, because since the creation of the world there has been systematic suppression of this truth. Only from the point of view of God's self-revelation can it be seen that this truth has been suppressed.
What then of the fact that "what can be known about God is plain to them, because God has shown it to them"? And more acutely, what about the fact that this has been plain "ever since the creation of the world" - i.e. not just in the beginning, but ever since?
I think Paul is saying: the information is all still there - but without God's special self-disclosure this will inevitably lead to ignorance, due to human sin. Again, I question whether this should be called 'revelation'. A process which cannot lead to anyone knowing God, but will through sin always lead to idolatry does not seem to me to deserve the label. At the very least, we must say that Romans 1 does not teach that people actually know about God - precisely the opposite!