This has just come up on the Gentoo mailing list. cdrecord DOES work with Gentoo and a 2.6 kernel, or you wouldn't have got the cover disc in the first place
The message is only a warning, and a very old one at that.
Try cdrecord dev=ATAPI -scanbus, although I just use standard Linux device naming, e.g. cdrecord dev=/dev/hdc blah...
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)