AP calls for "copyright" as an adjective: The disclosure was made in a copyright story. It sounds bizarre, I know. And almost every story about MP3 lawsuits comes over using "copyrighted." But change it (unless you have a compelling reason not to; if so, let me know).
Copyrighted is fine as the past tense of the verb. The band copyrighted the song. But under no circumstances is the past tense "copywritten," no matter what Missy Elliott says.