Rollup Pack 8 for Microsoft CRM 2011 shipped with a couple of nice JavaScript functions:
Xrm.Utility.openEntityForm(name,id,parameters)
and
Xrm.Utility.openWebResource(webResourceName,webResourceData,width, height)
Eg: //pop incident form with default values
Xrm.Utility.openEntityForm("incident", null, parameters);
Enjoy coding... :)