Hello, I've just reported a new Jira for excel export.
Jira 3834

I think the problem is in

JXLHelper.java 644

      case date:
         return new DateTime(column, row, (Date) data, cellFormat);

I think is the cast to (Date) that throws exception.

Should I filter in the model?

java.lang.ClassCastException: java.lang.String
     at org.jboss.seam.excel.jxl.JXLHelper.createCell(JXLHelper.java:644)
     at org.jboss.seam.excel.jxl.JXLExcelWorkbook.addCell(JXLExcelWorkbook.java:272)
     at org.jboss.seam.excel.jxl.JXLExcelWorkbook.addItem(JXLExcelWorkbook.java:514)
     at org.jboss.seam.excel.ui.UIColumn.encodeBegin(UIColumn.java:80)