There's this section in google's guidelines that says

There's almost nothing a competitor can do to harm your ranking or have your site removed from our index. If you're concerned about another site linking to yours, we suggest contacting the webmaster of the site in question. Google aggregates and organizes information published on the web; we don't control the content of these pages.

Many webmasters are afraid of the "almost nothing" phrase. Does this mean somebody can get your site delisted or have your site penalized by google? Apparently, there's one case where a spammer was able to get a website penalized by google.

It is currently being discussed in webmasterworld

Some notes to webmasters specially those who are running forums or sites where users can create their own accounts.

1. Check your site for spam user accounts that create links to spammer sites (these are usually in user profiles or in comments).

One command I use to check on these is

site:yourdomain.com viagra

It will list indexed pages of your site that contains that word.

2. If you run a wordpress, use akismet. That works for me.
3. Review your comments. Don't auto approve comments as it is a recipe for disaster.