Alright, as it is about less than a day less than the event, I would like some speculation. (I tend to be un-original, so I need somebody else's cool ideas)
What, not enough speculation for you in this thread already?
Okay, then, here goes [polishes crystal ball]:
I think that today's announcement will be, as the invitation suggests, the roadmap for application development and support on the iPhone and iPod touch.
I don't like to read too much into the pictures on Apple's invitations, but there do often seem to be clues in them, so on that basis, I would guess that we will be told that various enterprise-ready options for the iPhone will be available today or in the next few days. I would guess at improved Exchange server support, and possibly better VPN support. Possibly other stuff, but I will leave those with a better knowledge of mobile phone use in the enterprise to comment on that.
Jobs (or whoever) will say that the promised iPhone SDK will be made available in beta form for developers to download and start using. The SDK proper will be released in June, but the beta will give developers something to start using.
For the moment, apps will be distributed by means of iTunes. Each ADC member will have a digital signature, which will be added to the app at compile time. If anyone makes a malicious app, they can be traced. Apps can be free, or they can be chargeable. Apple will take a percentage cut of any monies from chargeable apps.
In the future, apps will be available "over the air", in much the same way as songs from iTMS are currently. This is the "software updates" element of the invitation. Possibly it will allow developers to make updates to their applications available over the air, or via Apple's own Software Updates mechanism.
In the meantime, several well-known Mac developers have had preliminary versions of the SDK for some months, and they've written apps that will be available for download today. Cue demo of a couple of games, plus one or two "sensible" apps.
And that's my prediction. That and that, 24 hours later, there will be a quarter of a million iPhone-based tip calculators on the iTMS.
Now, watch me be soooooo wrong.....