Assuming you're using QGIS >= 1.6.0. There is a geoalgorithm called "Polygons to lines""Polygons to lines" accessible via Vector > Geometry Tools > Polygons to Lines (otherwise search in the Processing Toolbox Ctrl+Alt+T), which will create a new shapefile with all the attributes of the original.
For PyQGIS it is "native:polygonstolines"
>>> processing.algorithmHelp("native:polygonstolines") Polygons to lines (native:polygonstolines) Converts polygons to lines. Converts polygons to lines ---------------- Input parameters ---------------- INPUT: Input layer Parameter type: QgsProcessingParameterFeatureSource Accepted data types: - str: layer ID - str: layer name - str: layer source - QgsProcessingFeatureSourceDefinition - QgsProperty - QgsVectorLayer OUTPUT: Lines Parameter type: QgsProcessingParameterFeatureSink Accepted data types: - str: destination vector file, e.g. 'd:/test.shp' - str: 'memory:' to store result in temporary memory layer - str: using vector provider ID prefix and destination URI, e.g. 'postgres:…' to store result in PostGIS table - QgsProcessingOutputLayerDefinition - QgsProperty ---------------- Outputs ---------------- OUTPUT: <QgsProcessingOutputVectorLayer> Lines 
