This is my blog post regarding directory submission and why I consider this a bad idea These days you can get teams in thirdworld contries building back links all day, nothing un-natural about this but the speed in which the backlinks are created can be a problem.
1) Backlinks that are created need to be on quality sites, yes there are companies even filling that gap but the grammer and the anchor text has also got to be right.
2) The rate of these back links going up, this too is important. Now what with social Media and blogging; backlinks will only get created faster. Backlinks going up by more than 300 - 500 per day is a fast growing rate.
My conclusion is that allot of people think more backlinks equals more page rank equals better site and more visitors.
WRONG
Two things I would do if I was google.
1) Ensure that I deleiver the best result for what the web visitor has searched for.
2) Sites need to grow naturally.
Number 2 is a hard one but looking at some of the small sites that have 4k backlinks is a great example.