Express AV, JamBox, Open GL 2, Widgets, iPod Software & Alicia Keys
Airport Express AV would be cool for streaming photo slideshows with music to a television. It may also do video but I would not hold my breath for HD. Very few people in Apple's market have plasma displays. Anyway, other than music videos and film trailers there is very little legitimate video out there so unless iTunes for Movies is also introduced there is no reason to rush to market something like this.
My money is on the infamous 'Asteroid' peripheral being introduced. Apple owns the trademark to the name Jam Box so maybe that is what they will call it. For those who do not know, this product is an analogue to digital breakout box for audio capture and output. GarageBand users could plug microphones and electric guitars into this device and record their sessions on the computer. In my opinion it will definately be a FireWire device since USB would need a seperate power supply to power microphones and FireWire can do more channels in HD audio.
Since this is a Developer's Conference I expect a lot of talk about OS X 10.4 but no introduction of OS X 10.5 yet; too soon for that. Maybe Apple will preview Open GL 2.0 for OS X which is rumoured to be introduced in OS X 10.4.4 or later. I am sure that a lot of time will be spent talking about widgets and encouriging shareware authors to make more of them.
As for the iPod, it would be great to see Apple make it possible for developers to write software for it. By now there are propably more active iPods than Palms in the world and there are a lot of applications that could be usefull in it. Something I personally wish for is a timer and more games.
Anyway, it will be hard to top the last Mac-show where iPod Shuffle and Mac Mini were introduced. Perhaps they should call Alicia Keys back for another live performance.