If you build a clean trusted brand many people will emulate your brand and leech off it. Everything from wrapping spam in the Google brand right on through to registering a domain name that sounds just like your name and doing mass email spam with it. You can't stop all of it (or even most of it) but you can defend yourself from a lot of it by:

1. Registering a couple of the more common alternative domain extensions (like .net and .org). This also has the benefit of locking out some competition if you own a keyword domain.
2. Adding the word the to your domain name and buying it
3. Adding an s to your names and buying the .com versions (the person who bought seobooks.com wanted $25,000 for it...I should have spent $8 instead)
4. If you own a 2 or 3 word domain name consider buying the version with a hyphen between the words
5. If you have an affiliate program you either need to actively monitor the search results or prevent affiliates from registering your brand in the URLs
6. If your brand domain is not generically descriptive then buy not only your brand name, but also buy the name with your brand name + your field of trade in it. Just recently one well known SEO firm was the victim of brand dilution due to someone registering theirnameseo.com and doing a massive email spamming campaign.
7. Given that the April 2007 version of Google's remote quality rater guidelines defined social sites as vital results if they rank then it is best to register your brands on the major social sites before others do. (Someone else is already seobook on twitter and I don't think it would be cheap or easy for me to change that at this point.)
8. For each major product launch or linkbait launch you may also try to get at least the matching .com name (advice I wish I would have gave myself in the past).

Some companies may also go so far as to check for other domain names containing their keywords, monitor recently registered names containing their trademarks, or pay a third party to do so. You probably do not need to go that far in most cases, but if you are going to put a lot of time and effort into building a brand then carrying an extra couple hundred dollars a year in registration costs is a negligible fee to help protect your offering from brand dilution from unsavory market competition.