I'm used activescaffold in countless rails projects. Rails 3, resets the clock a bit.
With each major revision of the framework, I always re-eval my toolkit to see what is a better or more up to date fix.
While I'm happy with the work I've done combining rails and silverlight, and I've also tried some Silverlight WCF RIA projects, a pure rails project has some appeal.
Looks like there is another choice. Instead of using activescaffold, look at NetZke.
It leverages ExtJs, and gives very nice gui, with better look and feel than activescaffold plus widgets. Overall, I'd say the base site for a business app in Rails 3 plus NetZke will look so much better than AS plus Widgets Menu I've been using a while for backend ops.
Of course if you want smooth, go silverlight for the Gui with REST interface.