The USCIS recently announced people could experience up to an 8 week delay in the delivery of their permanent resident card (green card) while they are actually updating the equipment that produces the green card or permanent residency card itself.
In the meantime the USCIS Field Offices will issue temporary evidence of permanent residence in the form of an I-551 stamp to applicants approved for permanent residence at the time of their interview. Just take your passport to your appointment. If you do not have a passport, you must bring a passport photo and some form of government issued photo identification to receive the temporary evidence of permanent residence.
Finally if the application is approved subsequent to your interview, by a Service Center or the National Benefit Center, the applicant should bring the above documents to an INFOPASS appointment to be issued temporary evidence of permanent residence in the form of an I-551 stamp.