I concur with Steve Rafalsky's answers (and highly recommend Riddlebarger's works on eschatology as well, including his addressing the "optimistic amil" term in his book,
A Case for Amillenialism).
While I don't find it compelling at all to read into Colossians 1:20 (even with verse 16 preceding it) a Postmil eschatology for a number of other reasons, it occurred to me that this verse may be relevant to consider in relation to it (similar to John 3:16 with "the world") in terms of what it's really about more generally and completely at Christ's consummation of His Kingdom on the Last Great Day (when there will no doubt be one world government under Christ with a willing people serving Him forever--Revelation 11:15, and the cosmos will "sigh a breath of relief" to no longer be darkened under the hold of the prince of this world and his minions in their various "reigns" over parts of the earth):
Rom. 8:19-23: For the earnest expectation of the creature waiteth for the manifestation of the sons of God. For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God. For we know that the whole creation groaneth and travaileth in pain together until now. And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.
While Calvin allows for this with a passing acknowledgment and goes into a long discussion of it relating to angels and men, the IVP-NB Commentary notes in part:
The climax of the paragraph comes with the references to reconciliation and peacemaking through Christ’s death. The opening words of the paragraph had stated that all things had been created in, through and for Christ. He is their Lord in creation. What is not spelled out, however, is what has happened to all things since creation: the unity and harmony of the cosmos have suffered a serious breach, needing reconciliation (cf. Gn. 3). It was God’s good pleasure to reconcile all things through Christ (2 Cor. 5:19). Heaven and earth have been brought back to the order for which God made them. The universe is under its Lord, and cosmic peace has been restored. Reconciliation and making peace (which includes the idea of pacification, i.e. overthrowing evil) are used synonymously to describe the mighty work which Christ achieved in history through his death on the cross as a sacrifice (Rom. 3:25; 1 Cor. 11:25; Eph. 1:7).
The commentary later also notes (I think relevant to the question and context as the impetus of this post):
Further, it cannot be assumed from this verse that all sinful men and women have freely accepted the peace achieved through Christ’s death. Although all things will finally unite to bow in the name of Jesus and to acknowledge him as Lord (Phil. 2:10-11), it is not to be supposed that this will be done gladly by all, and to suggest that v 20 points to a universal reconciliation in which every person will finally enjoy the blessings of salvation is unwarranted.
Lastly, I gotta give a nod to your user name, Colin!