Other quickly-mentioned Guitar Hero nuggets include the inclusion of a Metallica song in Guitar Hero III and plans to bring songs from the first two games to Guitar Hero III owners, presumably through downloadable content.
[UPDATE 2] Following the conference call, we followed up with Activision regarding the Metallica news. A representative confirmed not only that Metallica will be in Guitar Hero III, but that the song One will be a master track. One is from Metallica's "...And Justice For All" album released in 1988. It's also the very first song that Metallica produced a rock video for.