I'm about to get on a plane for Ajax World, but I just got word that the public alpha of Apollo is going to drop on the Adobe Labs site tonight. The current build is targeted mostly at Flex developers but has enough HTML support that Ajax developers should be able to dig in and get a feel for the platform.
The download will include the Apollo runtime, which is what users will download to run Apollo applications, the SDK for creating Apollo applications and Apollo Extensions for Flex Builder, an Eclipse plugin that works with Flex Builder and gives code completion and syntax highlighting. It also has a built in compiler so you can build Apollo apps and create them right from Eclipse. For those people not using Flex Builder, the SDK has a command line compiler. The book Apollo for Adobe Flex Developers Pocket Guide is available on Labs as a PDF download and is a good place to start.
In order to run Apollo applications, you'll need to download the runtime. Each individual application is packaged as an .AIR file which can be downloaded, emailed or transferred via any other mechanism. The typical Apollo application is between 500k - 1.5 megs, so not large. Double clicking on the .AIR file starts Apollo and installs the application prompting you with the typical install options. The whole process is quick.
If you're looking to check out Apollo applications I suggest the Finetune desktop which integrates almost seamlessly with their website, and Mike Chamber's Ascension application Teknision will be posting information on the Finetune Desktop on their blog sometime after the Apollo bits are out. Both deal with music but do a good job of showing off the features in Apollo as well as how it fits into the ecosystem. You can also play with Lee Brimelow's first Apollo creation.
Update: Adobe now has a press release out for Apollo, but I haven't seen the bits yet.
Update 2: Go get Apollo.