Two months since approval(I-485) and still no approval letter

Hi my I-485 was approved on 1/25/13 but I still haven't gotten my approval letter and now when I log on to USCIS to check my status it went from decision(approved on 1/25/13) to telling me that no information is available. What is going on? Is there any way my application could be denied even after it was approved.
USCIS got my correct mailing address, why haven't I received my letter?
Please help!!!!