So Apple is calling the press to an "event" where Steve goes on stage and says that the Nano is now taller and the Touch has a more rounded back. Then he says "thanks for coming" and leaves. That would be the last "event" the press would bother to attend. No, there is something music related that we don't know about that will be announced.
But then I could be wrong. Remember the "event" where Steve unveiled a very expensive powered speaker, and that was it. Could happen again.
So with this news, along with the comments of "something big" and presumably that "something big" is very likely related to "rocking on", my guesses are now:
1. Beatles (maybe a short-term exclusive). They're still big enough to drive some news stories, justifying the call for the press to show up (for more than just otherwise minor updates)
2. Maybe the

TV hardware update covering a few of the bigger wishes that have been floating around that device (though I would still bet heavily against the Blue Ray, DVR and central server storage wishes). I'm betting it will be 1080p capable (though little to no content to actually feed it 1080p video will show up for a very long time). Nevertheless a 1080p

TV is what I'd personally most like to see on the 9th (and I already have the existing model).
3. Steve is sick/Steve is retiring for health reasons. Famous CEOs would almost certainly call a press event to let the world know they are stepping down. And that would be big news making it worth the trip (for the press anyway). If this one, my guess is that it would not be immediate, a successor would probably be named (or a date would be called for naming one), and he would talk about one more MacWorld (09) in which something amazing will be rolled out, setting up a swan song. Like many, I hold a fair amount of Apple stock, so I hope this one isn't right.
I'm doubting the TV show rental concept, as that would potentially be fantastic for the consumer at the great expense of every cable giant. I just don't see even Apple being big enough to pull the revenue rug out from under the cable/satt subscription business, just like the iphone didn't pull the rug out from under the cell phone contract business. In both, there is just too much profit involved for the big communications companies to allow anyone- even Apple- to threaten their very lucrative, recurring revenue streams.
If it would happen, I would guess the rentals would be priced "too high" or we would have rentals with embedded commercials (to a chorus of "why do I have to pay to rent a show with commercials when I can get it for free?")