According to this Newsweek post, Guitar Hero Aerosmith means the band are tied into the franchise and won't be doing any Rock Band DLC any time soon. From the site:
"It's an exclusive deal for this game," Aerosmith publicist Marcee Rondon told Harris. We confirmed this with Tim Riley, Activision's vice president of music affairs, who told us through Activision PR that "I can say that we do have the band exclusively, and their catalogue should be exclusive to us beyond the one or two tracks they had licensed out to Rock Band before we made our deal."
More through the link.