Even i want to know
http://livedemo.exadel.com/richfaces-demo/richfaces/scrollableDataTable.jsf sample of selection management. http://jboss.org/richfaces/demos - link to source codes including java sources.
In the sample we just storing selection for showing in separate popup. But in the same way - you could assign it from action.
The only problem - it uses index'es of rows so being wrong after sorting./filtering applied. And solution for that problem shown there http://livedemo.exadel.com/richfaces-demo/richfaces/extendedDataTable.jsf?c=extendedDataTable&tab=usage - need to use simple model with provider which assosiates the rowKey's and objects in that model.