One of the things that makes Apollo such an intriguing technology is that it supports both Flash and HTML based applications. This means that Ajax developers can port their current applications over to Apollo without having to do anything with Flash. Ajax developers can then extend their applications with Apollo-specific APIs that open up Input/Output functionality and handle some of the security restrictions. Up until now, Adobe had kept the HTML engine a secret, but after sitting in on a press event with Kevin, they've announced which one they're using - WebKit, which is what the Safari browser is built on.
I was surprised that Adobe went with WebKit instead of the increasingly popular Gecko engine that Mozilla uses in Firefox. Ajax developers are very important to Adobe, and they want to make sure that the people building web applications are able to bring those into Apollo with very little extra work.
Both Gecko and WebKit are open source, and Adobe is planning to build off of the WebKit code base within Apollo. I was worried that Ajax applications would be slow to move over to Apollo because of the workarounds that are required to support Safari with most Ajax applications. However after talking to Adobe, it sounds like a lot of the issues are fixed in the most recent release of WebKit, so the transition for Ajax developers should be fairly painless.
Information on the HTML engine continues to trickle out, but if Apollo can get the kind of adoption that Flash has (or even close) it will give Ajax developers a unified platform to deploy Ajax applications on the desktop.