While they haven't been officially announced yet, three classic rock tunes have shown up on the leaderboards over at GuitarHero.com, which is pretty much a dead giveaway that they're on the way. One of the first Guitar Hero III download packs I feel is truly worth the cost, the three new songs are:
Journey - "Any Way You Want It"
Foreigner - "Jukebox Hero"
Boston - "Peace of Mind"