Sometimes once it's scanned at your local post office the tracking info. on the USPS website will say that you have received it. So you should still wait a day or 2 more before panicking.
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Sent: 8/30
Received: 8/31/12 12:30
G1145 Notification: 9/5/12 at 4 PM.
I-797C: Received 9/8/12
Biometrics appointment: 10/2 No walkin
Approved: 12/18/12
Nebraska