Apple (AAPL) CEO Steve Jobs is scheduled to return to the stage Wednesday for what has become a late-summer classic: Apple’s annual music-themed September special event — its sixth since 2005.

By tradition, this is when Apple unveils its newest iPods and the latest advances in its iTunes music store, giving customers and retail partners plenty of time to start planning for the upcoming holiday gift-buying season. In 2005 the headliner was the iPod nano, a product that was for a time — until the iPod shuffle displaced it — the world’s best-selling portable music player. The big news in 2007 was the arrival of iPod touch, a hit with students.

But the iPod product line is no longer the show stopper it used to be, and for the last couple of years, Apple’s September special events have left many in the audience — including journalists flown in from the East Coast — wondering why they’d bothered to come.

When Jobs was trotting out the fourth-generation iPod nano, the second-generation iPod touch and iTunes version 8 in 2008, Wall Street was so unimpressed that Apple’s share price dropped more than 7.5 points (4.7%) in the space of two hours. Last September the array of new products was so thin that the biggest news out of the event was the fact that Jobs, still recovering from a liver transplant, was healthy enough to take the stage at all.

Does he have enough material to put together a good show this year? Let’s review the rumors.

• A new iPod nano. Widely expected. According to several sources, it’s going to have a new, smaller form factor and a touchscreen instead of a click wheel. The biggest surprise would be if it can run some of the 250,000 apps written for the iPhone, iPad and iPod touch.
• A new iPod touch. A camera and a higher-res display would be a plus. Two cameras, including a front-facing camera for Facetime video chats, would be a big plus. A 3G version that works over the cellphone networks, as one rumor has it, would be huge.

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  A new Apple TV. This one is iffier, and doesn’t quite fit the guitar-themed invitation. But eventually Apple is expected to release a cheaper ($99, down from $229), iOS-based version of its set-top box, and this could be the week.

• A new version of iTunes. Rumored to double the length of song samples (from 30 sec. to 1 minute) in response to growing competition from Pandora and others. Could include social networking features such as sharing playlists with friends. Could be Web-based (so you can stream songs as well as  download them). Could include 99-cent TV show rentals (rather than $1.99 or $2.99 purchases) and, according to Bloomberg Businessweek, movies from Netflix.
• A new iOS for the iPad. A version of iOS 4 for the iPad — with multitasking, app folders and other goodies already available on the iPhone 4 — isn’t due until the fall, and nobody has seen a beta release yet. A long shot.
• A white iPhone. OK, maybe. But there’s not much of a gee-whiz factor in a product that’s a couple months late.
• A wild-card. A last-minute surprise — the “one last thing” that used to be Steve Jobs’ theatrical calling card — gets harder every year, given how porous Apple’s supply chain has become, but it’s still possible. Barrons‘ Erik Savitz suggested over the weekend that this could be the week Apple announces that it has finally cut a deal that would put the Beatles on iTunes. We’ll believe that one when we hear it.

If you take every rumor, leak and blurry prototype photo at face value, you are bound to be disappointed. Steve Jobs rarely delivers on the blogosphere’s every wish. But if you go with no expectations, you might just enjoy the show.

The invitation-only performance is scheduled for Wednesday morning at San Francisco’s Yerba Buena Center for the Arts. And knowing Apple, it will start promptly at 10 a.m. PDT (1 p.m. EDT). We’ll be live blogging the event from within the auditorium, the gods of Wi-Fi permitting.

See also:

• Handicapping Apple’s special events
• The case for a touchscreen nano
• 2009: No Beatles, just Steve Jobs and Norah Jones
• 2008: Apple fails to wow Wall Street

[Follow Philip Elmer-DeWitt on Twitter @philiped]