Try...

layers = self.iface.legendInterface().layers()

for layer in layers:
    layerType = layer.type()
    if layerType == QgsMapLayer.VectorLayer:
        # do some stuff here

Culled from consolidatethread.py from QConsolidate plugin.

Description of QgsLegendInterface object returned from legendInterface().

Try...

# substitute 'self' with 'qgis.utils.iface' when run from Python console
# 'self.iface = iface' would usually precede this command in your class 
layers = self.iface.legendInterface().layers()

for layer in layers:
    layerType = layer.type()
    if layerType == QgsMapLayer.VectorLayer:
        # do some stuff here

Culled from consolidatethread.py from QConsolidate plugin.

Description of QgsLegendInterface object returned from legendInterface().

Edit: added info on 'self' above.