For anyone that doesn't already know this, and I'm sure most people on this board do, it is cheap and easy to file for your own U.S. trademark online.
It costs $325 per trademark and you can fill out the form online at the government website in about 10 minutes and pay by credit card. They also have all kinds of online help if you need it.
Here is the link:
http://www.uspto.gov/teas/eTEASforms.htm
Think of it as very cheap insurance.