After doing some more thinking, I'll revise my guesses.
Likely: AirPort Extreme Update, either to the 802.11n draft (assuming it looks like it'll just be a matter of a firmware update to get it to spec eventually) or to the dual-channel 802.11g stuff that D-Link and Netgear make right now. Java 1.5 beta.
Possible: iBook update. Preview of next-generation Xcode Tools. List (no demos) of 10.5 planned features. Demo of QuickTime 7.1. WebObjects 6, or at least 5.5*.
Unlikely: PowerMac updates to either dual-core or 3 GHz. Mac mini update.
Very unlikely: 10.5 demo. Anything to do with iTunes or iPod.
*if that one, then here's some scenarios for that:
Likely: complete privatization of the Enterprise Objects frameworks to discourage hacks to use EOF and Cocoa/Objective-C (for those who need a bit more than CoreData can give them). A rewrite of EOModeler in Java

. Rewrite of WOBuilder in Java. Lots of defeated sighs from the few remaining WebObjects and EOF developers.
Possible: Apple admits it's mistake in choosing Java over Objective-C for WebObjects 5, announces WO6 (shipping later in the year) will have complete Objective-C support, in addition to Java. Updates to EOF, a major bug-fix and UI update release for EOModeler, and a WebKit-based WOBuilder. Huge cheering from the WO developers in the crowd (myself included)

.
Unlikely: Apple announces that WebObjects 6 will be based on Ruby or Python. Mixed reactions. Some self-defenestration due to frustration by those who don't care what Apple uses for WebObjects, just so long as it stays consistent.
Very unlikely: Apple discontinues WebObjects except for in-house use (iTMS, Apple Store) or raises the price back to $50,000 without any significant updates.