James Governor thinks that Eclipse might be the next client platform for applications, not just development:

I have long argued that Eclipse was going to shake up the smart client space. In fact, i was arguing that before IBM even began its rich client work under the auspices of the Eclipse organization. The thing is, Eclipse was never going to be just an IDE. It was never going to be a tool only used by developers or operators. Eclipse is a framework for everything, and when did a developer environment not drive an approach to the client, especially when it has so much cross platform widgetry?

He may be right; Eclipse is widely used. On the other hand, "extensibility" in Eclipse is limited to plugins - if you want real extensibility, you want the ability to sling code in the runtime. I did a screencast on that awhile back. I've added the feature discussed there to BottomFeeder since that cast, and it's available as an update. As to this:

Eclipse RCP may be nowhere near as rich as Flash, nor have even a micro-fraction the number of users. But it is an environment where tons of developers live every day. Where they can view source across any component. And as the Web has repeatedly shown, the richest experience doesn't always win. The sourciest does. Obviously AJAX building styles are in the mix too, for the same reason, and Flash and AJAX are showing nice familiarity.

The source for the entire Smalltalk image has always been available. Heck, as David Buck demonstrated awhile back, you can change the compiler (on a class by class basis) if you want to. Smalltalk may be a niche player, but it's definitely the "sourciest" environment around - and always has been.