Bad neighborhoods and link farms are in effect the same thing.
If you look at a site and it has themeindex.html in the main category URL, you know right then that the site is using Zeus. In most regards, this a considered a link farm and sites using tools of that kind are considered bac neighborhoods.
If you begin to capture links from sites that are spamming the search engines, they too are considered bad neighborhoods.
On to your second question.
PR has nothing to do with placement or link popularity.
PR is a valuation of the information you have.
PR is determined by the number of links coming to your web site from like-kind web sites compared to those that are not of like kind.
Ever since SearchKing got busted,
PR can not be inflated through links from non-like-kind web sites. It can only be increased by acquiring links from like-kind web sites.
It does not hurt to have a PR0 link to you.
Any links from sites with a higher
PR than your site can help increase your
PR.
Link popularity is based upon the links into your web site from other sites. It is not based upon the expectations of like-kind web sites.
If you would like more information on this, then let me get the date scheduled with Google for my interview of them on my radio show. After the show, I plan to have the transcript available for everyone to read.
If you have specific questions you would like answered regarding Google, let me know. I'll see if I can work them in.