Many sites who are working on link building often end up tripping googles BLOOP filter.
BLOOP stands for "backlink over optimisation penalty" which comes about when you do something while building your backlinks that exceeds the norm.
for example having all your anchor text the same is a common and very old one.
A much newer example of a BLOOP filter is discussed in googles white paper on temporal link analasis.
Esentially if you get to many links in a very short period of time these links get devalued and discounted with regards to helping you rank (although they still pass PR)
Google generates an average paramiter for the ammount of inbound links you get over a specific period and if there is a sudden spike in the ammount of inbound links it finds this triggers a BLOOP.
The best way to counteract this is to focus on high quality & high PR links, let the lower quality links come in naturally at a steady pace.
And if you fancy being creative find a site that is linking to you with good anchor text and not many links going elsewhere (or build one and host it on a different Ip range) and point all your low value links to that site.
The PR still gets passed to your site the link/s will gain strength over time as well.
Source
http://darksat.x47.net/topic/320.0.html
