The plug-in must support more browsers for applets to succeed
I know it sounds counter-intuitive to waste resources porting the plug-in to platforms that only have a 5% market share but I think this should be your not-so-long-term goal. I agree that the FireFox/InternetExplorer plug-in need to be *solid* before even considering this.
You need to realize that the primary reason that AJAX is the hottest technology around town is that it works across all browsers while delivering decent UI. Developers will think twice before migrating from AJAX to Applets if they hear that the plug-in only supports two browsers (regardless of the market-share that entails).
I think you could reasonably begin this effort with minimal work on Sun's part simply by open-sourcing the plug-in and encouraging the community to help you port it to their favorite browser. Browser zealots just love that sort of thing :)
Just my 2 cents.