"The courts read any failure to protect your trademark as good reason for the courts to not help you protect it later when you find the infringement significant. As the trademark attorney in the clip noted, Elevator, Thermos, Aspirin, and Trampoline all lost their rights to their trademarks, because they didn't "police" their trademarks."
From: http://stosselintheclassroom.org/video_activityMar08.html
(Important for startups:
http://blog.startupipservices.com/archives/top-startup-trade...
I'm sure you knew already, but it doesn't hurt to reinforce the importance of trademarks.)
Edit: dotBen made a similar point further down the thread