Aaron is asking the question (and giving a reasonably good answer), "Can a Site be Penalized for Links?" over at SEOBook. My personal opinion, however, is that... wait for it... "it depends."
There are some sites that can be Google-bowled out of the index via spammy backlinks, but most of the time, the folks you'll be targeting with these types of campaigns are those that already rank well at Google, have a history of strong trust and good links. With highly established sites, even those that may not seem particularly strong, link-muscling them out with blog comment spam and linkfarm annihalation simply isn't possible.
How do I know? I got to see the results of some very nasty black hats that targeted a site for "elimination." The results, despte more than a hundred thousand spammy links, was nothing (other than highly improved rankings at Yahoo! & MSN).
The sites that are vulnerable to bowling are those that are new, have just moved to a new domain (301'd), have recently changed ownership or have never had a strong hold on top rankings, despite being a few years old. Would I ever endorse this tactic? No way. But, I do think it's important to give it some recognition so the search engines can learn more about just how much SEOs (particularly on the black hat site) can exploit their algorithmic systems.
If you wanted to "help" someone in Google and "not help" someone in Yahoo and MSN, just cloak the links.
Food for thought for a new type of googlebowling tactics, along with that post on SEO Book and another one there, "Can I Get Penalized for Who Links at My Site?"... the way things are now, it's a lose-lose situation for the SEs as no matter what they do there are still ways to use it against others if not for your own (direct) benefit.