Let's see what I can quickly jot down before I go to work..I don't have my Septuagint handy, and there may be a translation difference in the verse from the website used by TomS.The website seems to list this verse as v22 while others place this verse as v21:http://www.usccb.org/nab/bible/psalms/psalm68.htm
Our God is a God who saves; escape from death is in the LORD God's hands.
God will crush the skulls of the enemy, the hairy heads of those who walk in sin.
Assuming the NAB's v22 is everyone else's Ps68:21, there are two ways to look at this:
--One is reassurance that God will be victorious over His enemies. This, btw, is also the usual understanding of Hab 3:13 which uses approximately the same verbiage.
--From this point we get to the related point of the events occurring during the Harrowing of Hell. A Resurrectional hymn sung makes reference to this with: Forseeing Thy divine emptying of Thyself on the Cross, Abbakoum (Habbakuk) cried out in amazement, "The dominion of the might did Thou destroy, O good One, as One omnipotent, Thou didst encounter them that were in Hades" (italics mine)
--On a different tangent, this can also be seen as spiritual advice. Fr. Eugene Pentiuc, Old Testament professor at Holy Cross, has been known to refer to verses similar to this (as well as Ps 137:9) as 'the difficult ones..' because they seem to rationalize killing babies and infants.
But that is only if you take the verses in their literal way. Metaphorically, the infants and babies to be dashed against the rocks can be seen as the darts thrown our way by the Evil One---the initial urgings of temptations that lead us to sin if we indulge them.
Thusly, Ps 137:9 talks about how we should control these urges and eliminate them. Ps 68:21 then assures us that with the Lord on our side our victory against these urges is assured.
Again, my apologies for not having all my tools handy for this, and these are just some quick and undeveloped thoughts.