I looked at Harold Hoehner's (he's a prof. at Dallas Seminary) commentary on Ephesians, which is quite extensive.

Obviously, there is quite a bit of discussion regarding the way Paul used Psalm 68, since he did make some changes (none of which, however, were inconsistent with the thrust of this psalm).  Regarding these "captives" I was pleased to see that we both came to the same conclusion:

"Who, then, are the captives?  From Ps 68 it is clear that they were the enemies of Israel who were defeated when Jerusalem was captured.  In Ephesians some have interpreted the captives:  (1) as the enemies of Christ, namely, Satan, sin, and death; or (2) as the people who have been the captives of Satan, sin and death, and who are now taken captive by Christ in redemption.  The first interpretation seems to be more fitting.  Christ had victory over Satan, sin and death and gives gifts of the Spirit to those who have been identified with him."  Harold W. Hoehner, Ephesians:  An Exegetical Commentary (Grand Rapids, Michigan:  Baker Academic, 2002), pp. 529-530.

This interpretation seems entirely consistent with Colossians 2:13-15.