When you want to get good rankings for a particular keyword in Google, it's common practice to buy lots of backlinks with that keyword as anchor text. However, I can't help but wonder ... isn't that the most unnatural thing that any search engine could ever find?

If lots of sites gave you links all naturally, the chances that they all used the same anchor text is rather slim. And when you factor in the fact that your anchor text is amost never your company name - that makes it near impossible.

It's one thing if a good chunk of people who link to DaniWeb do so using "DaniWeb" as anchor text ... but it's quite another if suddenly every major backlink I got had the anchor text "computer help".

You make a good point, and Im sure thats what sets of the "too much too fast" filter from G, penalizing for alot of backlinks too fast.

So maybe it's not just that they got too many too fast that set off the "this doesn't look natural" flag ... but rather the same anchor text too fast

Definitely, same anchor text should be avoided. And also do not point always to the homepage, that's also unnatural for google.

Infact Google penalizes such "Unantural Anchor Text" backlinking .I have come across with many sites who were using massive amount of backlinking across many website by paying their monthly fees but they landed in problem.

What Google likes and give weitage to a natural anchor text link from the website withs same subject relevency.

Use of varied anchor text is useful and suggested.Anything which is closer to "Natural" is the rewareded by the Google well

Very good point, which is why many SEO cats advise that you keep your anchor text varied. For example if you're targeting the keyword 'computer help' and use that as your anchor text in link exchanges, or purchase links, using anchor text such as 'computer help, pc forum' and 'help with computer problems' will still make the SE's pick up the words 'computer' and 'help' which will still help your SERP.

basically this is what has been focused in Latent Semantic Indexing-LSI (new google algorithm-though not implemented yet)..

Always try to use all variations of the keywords so that the overall effect is a unified theme for your site...

In LSI, the site with the strongest THEME ranks on TOP in comparison to the lever of keyword optimization currently...

Do you mean variations of the keywords in the anchor text for links and articles?

yea..even on the site as well.

Do not use the same keyword again and again..use the variations...

for in LSI, the system can understand the meaning of the words (Semantics) and corelate the association between documents....

for instance, mobile, light weight etc are related to laptops, so using these words along with laptop will make the theme of the document strong and you will rank on many keywords instead of just one-two keywords