sortable works only for scrollableDataTable, but not for rich:dataTable. This feature will be added later.
regarding the sorting of the scrollableDataTable, I think I found the following bug:
If you try to sort a column (by clicking the column header) containing entries of the type date, you get a NullPointerException in case one or more date-values are empty in the table.
I think there is no JIRA issue on that.