In the meanwhile mhlmi on #qtruby has created a stripped down example which proves that the bug has nothing to do with
bigdecimal.

---< test.rb >------ 
require "pp"
require "Qt4"

class MyModel < Qt::SortFilterProxyModel
attr_reader :model
   def initialize(*args)
      super(*args)
      @model = Qt::StringListModel.new
      @model.setStringList((0..10).to_a.collect{|i| "test #{i}"})
      setSourceModel(@model)
#       @dummy = Hash.new []
   end
   def data(index, role=Qt::DisplayRole, uff=0)
      return Qt::Variant.new unless index.isValid
#       item = @dummy[index.column] 
index.column ## 
      if uff > 100
         super(index, role)
      else
         data(index, role, uff+1) 
      end
   end
end
 
class MyMainWindow < Qt::MainWindow
   slots "test()"
   def initialize(*args)
      super(*args)
 
      @wid = Qt::Widget.new
      @lay = Qt::VBoxLayout.new(@wid)
      
      @model = MyModel.new
      table = Qt::TableView.new
      table.setModel(@model)
      table.setSortingEnabled(true)
      btn = Qt::PushButton.new
      connect(btn, SIGNAL("clicked()"), self, SLOT("test()"))
      @lay.addWidget(table)
      @lay.addWidget(btn)
 
 
      setCentralWidget(@wid)
      resize(400,900)
   end
   def test
      GC.start
   end
   private
end
 
 
$KCODE='u'
Qt.debug_level = Qt::DebugLevel::High if ARGV.shift == "debug"
app = Qt::Application.new(ARGV)
mainWin = MyMainWindow.new
mainWin.show
app.exec
---< EOF >------

Using Qt::Internal::setDebug(Qt::QtDebugChannel::QTDB_GC) it is clear that the QModelIndex instance is the "broken"
object.

Sorry for fileing 2 bug reports, but i wasn't able to add a comment to the former one (ERROR).