You’ll want to watch this YouTube video featuring Matt Cutts from Google talking about how Google feels about your competitors trying to sabotage your Google.com organic placement by setting up spammy sites to link to your website in a effort to push you down in the results if they can’t supersede you. (http://www.youtube.com/watch?v=HWJUU-g5U_I&feature=em-uploademail)
Typically small to moderate sized businesses will never run into problems such as this. However for very competitive industries, you may actually run into this problem. Google has created the disavow tool to assist in re-mediating these types of problems.