You may be found to have abandoned your permanent resident status if you:
.......
-Fail to file income tax returns while living outside of the United States for any period
-Declare yourself a “nonimmigrant” on your tax returns
Cited from:
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=3f443a4107083210VgnVCM100000082ca60aRCRD&vgnextchannel=3f443a4107083210VgnVCM100000082ca60aRCRD