First I suppose we should look at what all in all is thought to mean. It isn't what I thought at all! It is a sort of 'all things considered' meaning - yech. All in all as a 'nothing' adverb is not what I have in mind.
Here's the text under consideration - just in English (NIV) - with the lead up to it.
But Christ has indeed been raised from the dead,
the firstfruits of those who have fallen asleep.
For since death came through a man,
the resurrection of the dead comes also through a man.
For as in Adam all die,
so in Christ all will be made alive.
But each in his own turn: Christ, the firstfruits;
then, when he comes, those who belong to him.
Then the end will come,
when he hands over the kingdom to God the Father
after he has destroyed all dominion, authority and power.
For he must reign until he has put all his enemies under his feet.
The last enemy to be destroyed is death.
For he "has put everything under his feet."
Now when it says that "everything" has been put under him,
it is clear that this does not include God himself,
who put everything under Christ.
When he has done this,
then the Son himself will be made subject to him who put everything under him,
so that God may be all in all.
So that God may be all in all. Literally 'the all things in all' The second all is dative. Both are plural. The first is neuter. God is singular (you better believe it!) But God will be all things (some singularity!) to all - presumably all people. Or all living. There is some similarity to the thought in Hebrews where the same Psalm 110:1 is used.
But I promised to say what this means to me.
'He/she is my all in all' is a phrase that lovers use of each other. This is how I understand the all in all in this passage. It is the act of ultimate love that comes when the distracting and rebellious principalities that have some sway in this (my) life are brought to heel. There are many who believe this cannot happen but they should hold their surety more loosely, for in Christ all things are possible and all things can be brought into definitive realignment in our tortured lives. Our king becomes thus our all in all and in turn, we come in him in his submission to God, and God is then our all in all. This pattern will have, I expect, many forms of which Paul has expressed an ultimate intent in this passage.
People explain this passage as part of the eschaton - the last things. I suggest we not wait for the end to discover the reign of Christ. We learn to rule with him through the consecration that he has effected for us in his death. It is quite possible to use his death now as a means to our own life. We are told/commanded/invited to do so. Take all those verbs positively. We will make mistakes too and need his presence in mercy as well as transformation. We may hear his voice - "well done, faithful servant, you have overcome through my blood". And we may hear in our weakness - "we don't do things that way". His "we" is inclusive and also invitational. All the training he gives today, he also gave to his Elect people in the past - hence the importance of the Hebrew Scriptures. I was desperate to learn them all at once. Cool it. Read large chunks but also read slowly. One of the reasons I translate is to slow myself down and enjoy the ride.
The discussion in the blogosphere is here, and there, and everywhere and so on.