message from USCIS

Just receive the following message from USCIS :

On November 29, 2007, the post office returned our last written notice on this case as undeliverable. This can have serious effects on the processing of this case. Please call 1-800-375-5283 to update the mailing address so this notice can be re-sent.
TX center
PD03/2006
I485 07/28/07
Finger pring 11/04/07

What is the notice could be? Green card?!