I just read this post written by Steve in May 2008.
"My first thought was, how in the world did Moldova come to be ranked 37, above nations like Nigeria, Australia, South Korea, etc.? They've beaten Kazakhstan (122), Malta (135), Bosnia (62), Hungary (57) and Latvia (72), and had a draw with Turkey (25) in the last couple of years so they're not exactly beating world powers. FIFA rankings just don't make sense to me sometimes."
Moldova also won against Bosnia and Herzegovina - ranked 25th at the time of the match. See the August 2007 ranking. That win and the ones against Hungary and Malta (all three in the EURO 2008 qualifiers) brought them more than 50% of their total points.
Nigeria had an unsuccessful ACN 2008. On top of that, they failed to play 5 matches between May 2007 - May 2008. They only played 4 matches and that's basically another loss for them as FIFA's ranking algorithm requires a minimum of 5 matches in every 12 months frame.
Australia and Korea Republic - it's basically the same thing. Both rich federations, they can afford to play friendlies against top opposition and most of the time, they lose - that's 0 points and the average goes down.
Moldova on the other hand only plays against average to weak opposition and will get the win more often than not.