The current Quantum GIS master (i.e. the latest development version) supports now loading and saving of Styled Layer Descriptor (SLD) styles in the layer properties dialog.
The QGIS API has been extended by the new methods saveSldStyle and loadSldStyle in QgsMapLayer and writeSld and loadSld in QgsVectorLayer. Currently these methods are not yet available in the Python API, but it is simple to add them in the corresponding SIP files.