I just returned from a trip to Mexico with my family. My wife is a lawful permanent resident. I have written several articles before on issues related to foreign travel and LPRs and the requirements for reentry. Although technically, one only needs a valid and unexpired lawful permanent resident card to return to the U.S. as a lawful permanent resident, on this trip the immigration officer requested to see my wife's passport.
So, for those of you LPR's traveling abroad its probably a good idea to make sure you have a valid passport to take along with you as well.