First, Silverlight and Adobe AIR really are not related at all, and don’t target the same things. As I have said a million times before, if anything, the comparison is between the Flash Player and Silverlight. However, that is not what jumped out at me.
Prism isn’t a new platform, it’s simply the web platform integrated into the desktop experience. Web developers don’t have to target it separately, because any application that can run in a modern standards-compliant web browser can run in Prism. Prism is built on Firefox, so it supports rich internet technologies like HTML, JavaScript, CSS, and <canvas> and runs on Windows, Mac OS X, and Linux.
And while Prism focuses on how web apps can integrate into the desktop experience, we’re also working to increase the capabilities of those apps by adding functionality to the Web itself, such as providing support for offline data storage and access to 3D graphics hardware.
Do you see where I am going with this? You could describe Adobe AIR in exactly the same way (just replace Prism with Adobe AIR and Firefox with Webkit).
So, I guess the thing I found odd was Mozilla appears to be building something very similar to Adobe AIR (which is fine and cool), but somehow it is inherently good when Mozilla does it, and inherently evil when Adobe does it.