An interesting note about copyright.
The Walt Disney Company, around 1980-1985 encountered a legal snafu I'm sure none of them planned on.
The rights the the image of Mickey Mouse became public domain for about two months before they caught on.
How this came about, I'm not too clear on - but I do remember a BURST of non-licensed Mickey material (including some adult stuff) that surfaced during this time. Legally, Disney had no power to stop this and, of course, regained the rights to their property.
The courts, after a flurry of Disney-initiated lawsuits, also went on to pass a new law on copyright. What that law was, I'm not too sure, but I think it has to do with the USAGE of a copyrighted property.
Something along the terms of if you abandon a copyright for long enough, or if you don't protect a copyright once it's created, then you can lose it making the work public domain until it's protected again.
If anyone finds more information on this, I'd be very interested in knowing more about it.
Trademarks and patents are always the best way to go - there are a few nasty loopholes associated with copyrights (especially with the registered mail method that some artists and musicians use).