Over at Comics Should Be Good, Brian Cronin has a very good Q&A about the whole trademark thing.
Had I known that Brian was going to write all that, I probably wouldn't have bothered with my own little rant in the first place, as he does a much better job.
As a side point, if you're ever wondering why sometimes Marvel or DC will produce a one-shot or a mini-series with a b- or c-list character, it is often done in order to preserve the trademark. Unlike patents, which last for 20 years from the time of issue, or copyrights, which last Life + X number of years from creation (until/unless Congress extends them again and again and again...), trademarks have no set time limit before they expire. A trademark can theoretically last forever, as long as the owner continues to use it. But if the owner goes a certain period of time without using the trademark, it becomes abandoned and anyone can use it. So if every few years Marvel puts out a comic with the Colossus logo slapped on the cover, they continue to protect their trademark.
Team-up comics used to be great for this. Put out a comic with Batman, Superman or Spider-Man teaming-up with your lesser known characters, slap their logos on the cover, and you're all set.
(Again, I am not a lawyer, just a librarian.)