Ok - just to clarify things... the I-89 is the small form that Immigration Officers use to capture an applicant's index finger print for the GreenCard. This form is filled out when you hand over the approval notice or are approved at the local district office.
So, what Nebraska was telling you is that they have recieved your I-89 card and that they have a backlogg reviewing them and ordering GreenCards.
I suggest you take that letter and your approval letter to your local district office and get a temperary GreenCard. Once you have your temp. card, you should send Nebraska a request for update every 2 months or so until your plastic card arrives.
Good luck! I am sure you will hear some positive news real soon.