Useful links: http://labs.adobe.com/wiki/index.php/Apollo:Documentation http://labs.adobe.com/wiki/index.php/Apollo:developerfaq
NB: "Apollo 1.0 will not have built in support for communicating directly with databases. However, it will be possible to write Database drivers in ActionScript (leveraging binary or XML sockets), which would allow Apollo applications to communicate directly with a database (both local and remote)."

XHR: on success, on failure, pause an xhr and come back to it later.
Yes, asynchronous actions are supported.

Timer/Timeout.
Yes.

History/state (back button)
If you build one.

Off-line Storage?
Yes.

Interaction with the Desktop applications?
Yes, you can tell the OS to launch apps for a specific file.

Extensibility.
"Will developers be able to extend Apollo with native code?
No. This functionality will not be available in Apollo 1.0."

Animation.
Yes (Flash).

Number of Items that can be displayed in the interface without causing memory or computation problems.

Amenable to other technology mashups/ javascript frameworks: Can it be used with other javascript frameworks.
Yes. Actionscript to Javascript communication is possible.

Support for stateless messaging? -scalability
Yes.

Usability? Bookmarking/Layout font-size changes
Must be custom built.

Browser Portability
Runtime currently supports Windows and Mac OS

Start Time? How big are the javascripts/swf files....
Small, although runtime file is several megabytes.

TESTING and Debugging tools. How can you test the GUI interface?
Flash Debugger, Apollo Debug Launcher.

Will the generated HTML be portable and what happens when a new browser comes in?
n/a

Remote Process Calls? Different from JSON and XML
Yes, through Flex.

Batch XML Http Requests?
Not sure.

Language
Flash/Flex & HTML/Javascript (Must download Apollo runtime)