This is always a bind in whatever GIS package I seem to use - probably because I forget to concentrate over my projection details. However the easiest way to do it is to use fwtools. Download the pack from fwtools.maptools.org and move your data (in this case a shapefile) to the fwtools directory to make it easier.
Load up the FWTools Shell and type the following to convert a shapefile from OS National Grid to Google Maps EPSG:900913
ogr2ogr –f “ESRI Shapefile” –t_srs EPSG:900913 output.shp input.shp
Load into geoserver and test in a google maps page and you're done.