GROKLAW has an interesting post about a recent NY Times story reporting that Leo Stoller, who by appearances is an expert at sending trademark infringement threat letters, was able to trademarked the word relatively common word "stealth."
As you can see, it appears that Mr. Stoller aggressively defends his trademarks and does very well against people who don't fight back. Give Mr. Stoller credit for defending his marks.
Groklaw provides two links for further information about the tactics used by this gentleman, once his marks have been approved.
Here's an explanation on how trademark law does work on Bitlaw.