For example, how much weight does a footer link on a large forum have...say...5,000 backlinks from one domain. I know that multiple links from the same URL are deminished, but to what extent?
I can't speak definitively but I've given run of site links from one of my domains to several brand new sites. Their PR went from "none/grey bar" to P4 in the next update. The target sites were doing minimal link building so I'm pretty sure the PR boost and the indexing was from my site. [edit] The site I'm referring to has over 250,000 indexed pages and gets visted by the big three 3k pages a day. ???
From what I know, the relevancy of the site linking to your site is the most important factor. As for the number of links, I don't think it hurts if you have several links from the same site.