As I understand it this would be achieved using call-backs. That positions JSF as *another* hurried (read harried?!?) catcher-upper/filler technology.
Also, as someone else mentioned previously, the ball-park is once more being opened for vendor-specific lock-in around various components. Historically this has shown me that it impacts cross-platform capability.
Clearly these are issues around JSF. Even more concerning for me at this juncture is that JSF brings me no closer to a consistent model for coding/architecting the UI, business logic and data logic. Acronym + paradigm = relearn for each app layer, per vendor!
Losing a little faith here :-)