I just experiment to try create a certain keyword to certain blog address. My blog address are http://indonesian-food-forage.blogspot.com/2009/06/tomato.html, http://indonesian-food-forage.blogspot.com/2009/06/papaya-tree-and-fruit.html and http://indonesian-food-forage.blogspot.com/2009/05/potato.html, using different keyword for each blog address. tomato, papaya and potato.
For tomato I have link from many article and web from internal and external link, but the rank still don't increase. Strange condition happen to my other blog address like on papaya and potato, even I don't really serious to create a link to this blog address with certain keyword but the rank have increased itself. May this called as effective link by google, we can create huge link from many website but don't increase pagerank, even many links come to.
So this why link from related website or related blog article will valued than from many unrelated article links. If we create too many link on certain site also will decrease the quality of link because the link value also decrease.