Osmarender / Tiles@Home has been shut down as of March 2012, therefore the olwidget.js#L86 raises an error and no map is rendered with the default options.
In olwidget.js I replaced this:
return new OpenLayers.Layer.OSM.Osmarender('OpenStreetMap (Osmarender)');
return new OpenLayers.Layer.OSM.Mapnik("Mapnik");
It seems to work.
Thanks. I've removed osmarender in the latest master. Good catch.