One thing I see a lot of is the fact that Nyan Cat is in several other commercial works. This is absolutely true, but it is irrelevant for two reasons. First, the creator of Nyan Cat has been approached by several companies, who then ask permission to use that likeness in their works. Money changes hands, and the creator IS credited. Neither is the case in the case against Warner Brothers. Second, this is a lawsuit against Warner Brothers. What other games and/or properties have done is not the case in question.
I also have seen a lot in this thread with people whining about "Precedent". This is completely irrelevant, ignorant, and pedantic. No precedent is ready to be set, except that maybe small content creators will be able to protect their creations from theft by large corporations. This type of suit is not new in any regard. Apple is suing Samsung over some design similarities (rectangles with rounded corners, look it up), the lawsuits against Youtube for parody works (Viacom Vs. Youtube). This suit is upheld the precedent of many, many lawsuits under the Digital Millennium Copyright Protection Act. All this said, I support the suit. Back to my first point, if other companies recognized the copyright held by the creator, and the fact that copyright is opt-out, not opt-in, it is clear that WB either did no research before using the property, or knowingly used the properties without credit, or payment.