What things do you mean? Can you be more specific than RESTFUL webservice integration - which, btw, you can do with GWT already with RequestBuilder.
I have a great respect for the Spiffy UI team - Zack and Bess have written some awesome articles that truly inspired me in working with GWT:
http://www.zackgrossbart.com/hackito/antiptrn-gwt2/ These days, I don't use any GWT RPC and I don't even have any GWT widgets, but GWT is fantastic as a tool - or as Zack says, the 'glue'.
As I understand it, one of the big pushes for GWT 3.x is modularity - so we want less stuff in the core, and more libraries that can be used. So, what is needed for GWT to better support 'modules' like SpiffyUI?
Tom