To be honest I don't use the @UiHandler annotation much, because as you said, it starts to muddy the lines between the View and the Presenter. It's totally fine to use, and a great shortcut if you don't particularly care about sticking to the pattern, though.
The presenter.onAboutClicked() route is definitely an option, but then you might as well define the handler in the presenter in the first place.
I tend to use the @UiHandler approach only when the Handler does view-specific stuff, like adding style names, etc. For other purposes (adding validation, etc), I stick with the old approach of adding the appropriate Handlers in the Presenter.
I'd recommend reading through one of the many threads on GWT Google Group about MVP and UiBinder, such as this one.
or if you are looking for a real implementation on how to make it work check out this blog post, I have to say that up until I found it everything else was still noise in my learning.
gwt 2.0.x is a great improvement on gwt 1.x but I still believe google has some way to go with their documentation and guidance since as we have seen with uibinder and mvp, it leaves a lot to the imagination on how to make things work.
hope the link shades a little bit more light on what u need.