[Kristian Joensen]

Well lets put it this way:

1)If you HAVE got a written permission then you CAN use the character within the terms of that agreement.
2)If you don't have such a permission you may or may not be able to use the character depending on the individual circumstances.

In the US system there is a thing called fair use some uses of the Dopefish may fall under that. But there is an other area of law that is interesting in the case of the Dopefish that is the matter of parodies and the like.

Such things are a matter of how the court system ways copyright law versus the 1st amendment right of free expression. I think it is likely that the many appearances of the Dopefish in many games mostly fall under that category.

Edit:

It is also important to note that the copyright of the actual dopefish art and the issue of the trademark on the character are two very different things, since trademark law and copyright are two very different bodies of law.