Thanks Chris, you got it right - it was the # that was the problem.
However, %35 didn’t produce a map, but the hex value of 35 or %23 worked.
http://64.119.128.70/geoserver/wms?request=getmap&version=1.1.0&service=wms&SLD_BODY=<StyledLayerDescriptor+version=%221.0.0%22+xmlns%3Agml=%22http://www.opengis.net/gml%22+xmlns%3Aogc=%22http://www.opengis.net/ogc%22+xmlns=%22http://www.opengis.net/sld%22>massgis:GISDATA.C21E_PT<UserStyle+xmlns=%22http://www.opengis.net/sld%22><PropertyName+xmlns=%22http://www.opengis.net/ogc%22>SHAPESQUARE<CssParameter+name=%22fill%22>%23FF00005&bbox=249667,918553,250454,919353&format=image/png&width=400&height=400&SRS=EPSG%3A26986
Aleda Freeman
MassGIS
Office of Geographic and Environmental Information
251 Causeway St. 5th floor
Boston, MA 02114
phone: 617-626-1193
fax: 617-626-1249
http://www.mass.gov/mgis - MassGIS
http://maps.massgis.state.ma.us/biodiversity - Biodiversity Days