Is the commandment to multiply, and replenish the earth in effect today? Adam and Eve were commanded to multiply and replenish the earth.

Genesis 1:28-30
28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
29 And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat.
30 And to every beast of the earth, and to every fowl of the air, and to every thing that creepeth upon the earth, wherein there is life, I have given every green herb for meat: and it was so.

The same commandment was also given later to Noah and his sons.

Genesis 9:1-3
1 And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth.
2 And the fear of you and the dread of you shall be upon every beast of the earth, and upon every fowl of the air, upon all that moveth upon the earth, and upon all the fishes of the sea; into your hand are they delivered.
3 Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things.

In both cases the commandment to multiply and replenish the earth is followed by a commandment to have dominion over the earth (perhaps the two are related, but that's for another discussion). I point that out because surely the commandment to have dominion over the earth is still applicable to us. Does that mean the commandment to multiply and replenish the earth is still applicable? Its pretty significant if having a family is a commandment as opposed to being a suggestion or just optional.
A commandment is in effect until it is fulfilled. God has not told us it is fulfilled, so it is in effect unless we believe it to be fulfilled to His satisfaction. 

Personally I don't believe it to be fulfilled. All the "over population" we see is simply a mismanagement of resources. There is plenty of livable earth and resources available. Not that replenishment requires exhaustion of resources, but our level of waste is appalling, and better urban planning could improve the living conditions where population is high. 

I believe the commandment to have domion over the earth includes a stewardship to care for it, which clearly we can do better. Perhaps that is why the two commandments were given together. As we do better with our stewardship to care for the earth we can do better to replenish the earth.

Searching around for any Christian statements on the commandment to multiple and replenish in modern times I found the following:


  We declare that God's commandment for His children to multiply and replenish the earth remains in force. 


-The Family: A Proclamation to the World
  The First Presidency and Council of the Twelve Apostles of The Church of Jesus Christ of Latter-day Saints September 23, 1995

As well as the Catholic church stance on birth control:


  . . . it is always intrinsically wrong to use contraception to prevent new human beings from coming into existence. 


Wikipedia summarizes other Christian views on birth control, which go both directions. Of course Birth control isn't exactly what you asked, but they are related.

No doubt someone could find links to contradict me. I guess it comes down to your personal beliefs. 

My personal belieft is we are each to have as many children as we can reasonably support and care for. I have four children. My wife an I believe any more and we would not be able raise and care for them effectively (but that may be a failing on our part). I have friends with 6 or even 8 children who do quite well as taking care of them.

As an aside: It continues to be important that we specifically as Christians multiply and replenish the earth, and raise our children as good Christians to be a force of good in the Lord's work. To stop having children because one believes the world to be flawed is to give up hope of improving the world. Children raised up with good Christian values are the greatest hope for a better tomorrow.