This is a good post on the basics of trademarks by Linda Joy Kattwinkel (via AIGA CPM). There is a marked difference between copyrights and trademarks, and all designers should be well aware of the implications of each.
From the article:
“Unlike copyright, ownership of a trademark is not about who created the mark. Trademark rights are created by using the trademark for goods and services in commerce. So when you create a logo for a client, your client will own trademark rights in that logo as soon as she starts using the logo on products or promotions for her company.”
Also, if you’re researching available trademarks, be sure to check out the Trademark Electronic Search System (lovingly referred to as TESS).