Apple has sent out invitations for a music-related event next week, and the smart money is on new iPods.The invitation, titled "Let's Rock," went out Tuesday morning inviting media to the Yerba Buena Center for the Performing Arts in San Francisco next Tuesday at 10 a.m. PDT. Rumors have been building for weeks that Apple is set to introduce new iPods at what will be its fourth September iPod event in as many years.We'll be there, and we'll be live-blogging the event, so make sure to come back here next Tuesday.