iPods are a given, and likely to be the thrust of the event. Apple has refreshed the line every year in September since the iPod's introduction. We're guessing a more iPhone 4-like touch with a camera and possibly a retina display, perhaps on a higher-end model.
Also expected is a new iTunes rental option for TV episodes. As of this writing, TV shows are available for purchase at US$1.99 each. Bloomberg has suggested that Apple will add a rental option for $0.99 a pop, with new episodes becoming available just 24 hours after their original air time.
The wildcard is a new Apple TV (or iTV) running specialized apps on the iOS. It certainly doesn't fit the music theme implied by the invite, but who knows. Perhaps Steve has one more thing up his black sleeve. In any case, visit TUAW on the 1st for our complete coverage.