I've put together a page to install AgileAgenda using SWFObject with the ExpressInstall feature and the AIR Installation Badge.
This means people with a Flash Player less than 9.0.115 should be able to first upgrade their flash player, and then use the easy badge installation method for AIR + the application. I gave it a try on Firefox + Safari on OSX and IE + Firefox on WinXP, all of them with a 9.0.47 flash player and it all seemed to work well. The code also displays a message suggesting people install Flash player or install the AIR application manually if they don't have any version of Flash.
I've put together a small archive of the necessary files to make this work. It contains files from the swfobject guys released under the MIT license, and you can consider anything I wrote to make this work also under that license (which allows you to pretty much use it any way you like).
Hope this helps some people.