I've seen a lot of sites like that. Then the next PR update comes a long and it drops. I had a backlink from a PR6 and I asked him how he did it since he literally had 2 backlinks, which were PR0. Made no sense to me. Then after the latest PR update it's down to 0.
I agree with jumpness about the pr changing in the next update. At least that's what I have seen. IMO usually what happens, is that the quality of the links gives it a nice boost for the time being. Then it is up to the site owner to continue off page SEO or they lose some pr in the next update.
Do you think you have to keep getting links all the time? E.g. If you are PR3 and have 500 back links, if you don't get anymore could be drop?
I think it depends, but PR fluctuates from time to time. The quality of your links also changes from time to time (link wheels for example are said to not be as effective any more) For that reason, I think link building should not stop.
there are several factors to decide the PR of a website not only back links. your content and number of posts are also the factors behind PR.
a link wheel is the use of 5 social media sites to form a "wheel" back to your money site. for example, you write 5 variations of one article and post it on xanga, blogger, livejournal, myspace, and a 5th site. Each of those articles has 2 links. One link points to the next web 2.0 property and the other link points back to your money site. You continue doing this until the 5th web2.0 site points back to the first, effectively creating a "wheel" link wheels are not as effective as they once were (according to some ppl so don't quote me on this). You can see several variations of link wheels to make them more effective etc.