2.0 Blogs are one type of link. You also have blog comments, URL Shortening, Wiki Links, Track Backs, and so on. You have plenty of options. The problem is some are lower quality and do not exactly work best long term.
Your tier one are high quality. What points to your tier one, or tier two or three, can be any other type of link, namely blog comments because they are cheap and easy.
So tier one links point to money site.
Tier two links point to tier one, gives them juice. Think medium quality like Wiki links.
Tier three is normally spam. You throw everything and anything at your tier two links.
Keep in mind, this applies to a traditional tier model. A lot of variation exists.