USCIS - How Do I Get A Travel Document?
If you are not a U.S. citizen, you may need permission to return ... registration receipt card (Form I-797) or temporary evidence of permanent resident status. ...
http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=17be96981298d010VgnVCM10000048f3d6a1RCRD&vgnextchannel=828807b03d92b010VgnVCM10000045f3d6a1RCRD