Happy birthday, Apple Store! Thursday marks the 10th anniversary of Apple's retail juggernaut, which has ballooned from a single outlet to 323 global locations, raking in $3.2 billion last quarter. The Apple rumor mill says there'll be a big celebratory surprise this weekend. The evidence: Reportedly, each Apple Store is asking 10 to 15 employees to work a special overnight shift on Saturday. It's said they'll hang up black curtains, and will have their cell phones confiscated so they can't leak the big news. Whatever could it be? Here, four theories:

1. Apple Store 2.0
I predict "a major makeover for the Apple retail store experience," says Marc Gurman at 9 to 5 Mac. According to my sources, that means new areas called "Startup Sessions," interactive iPads to replace paper signs, and beefed-up sound systems. Apple is also due to release a new Apple Store app that would immediately register that a shopper has entered a particular store, and ask her what sort of service she needs. I can see it, says Jared Newman at TIME. "A store revamp would give people something to do on an otherwise lazy Sunday — the perfect activity for Apple worshippers."

2. A new product
"Apple could be about to unleash a shiny new toy on an unsuspecting world," says Kate Solomon at Tech Radar. But don't expect a new iPhone or MacBook Pro. Apple wouldn't bust those out "without a big media event and all the hoopla that those entail." A new basic MacBook or Mac Mini are the "leading candidates," says Newman. Those were last refreshed a year ago, and are due for an update. But "I wouldn't rule out a surprise release of OS X Lion," either.

3. The iCloud
Maybe the long-rumored iCloud service is finally here, says AppleInsider. The timing would make sense for this streaming and storage system for music and other media. Recent reports have said that Apple is done working on it, and has made deals with several major record labels.

4. Nothing special
Let's not set ourselves up for disappointment, says Daniel Ionescu at PCWorld. "While this might be a good moment to launch long-rumored Apple products, such as a TV set, or a free MobileMe service, Apple is probably just preparing some special discounts, or they're just redecorating." It's certainly possible that all this hype "is hogwash and nothing's launching on Sunday at all," says Newman.