All drawed, no Potrace. More or less 4h and blurs, and blurs, and more blurs...

Image or reference:
http://www.hachemuda.com/imagenes-bertus/bombilla.jpg
SVG: http://www.savefile.com/files/1029149
More information here (in spanish):
http://joaclintistgud.wordpress.com/2007/09/04/bombilla-dibujada-con-inkscape/
Someone wrote about it (in spanish):
http://www.berriart.com/2007/09/06/%c2%bfhasta-donde-llega-inkscape/
I hope you like it