Wicket can render any kind of XML (including non-compliant dialects like HTML). So far we only ship a set of components that work with X/HTML, but a few people have written alternatives such as WML already. The nice thing about Wicket is that it is so easy to write new components.
In JSF each component can have more then one renderer. So you can have a JSF app render into HTML/WML/telnet without changing a line of code and if you only use components that provide all those renderers. Keeping in mind that we are talking strictly about UI here, do you think that pages you layed out with HTML and a 1024x768 screen will look good on a tiny phone screen in WML without any intervention? Do you think your app will be as usable over telnet as an app written with telnet in mind?
I think this is a feature that JSF proponents always shout about when comparing with other frameworks because JSF is the only major tech out there that has it. But then you have to think WHY it is the only major tech that does?