Unfortunately, it looks like TSC is not sending out the extension letters. Whether they will eventually or not is still an unknown. So far, those of us who have filed at TSC since early February - except for one derivative individual - have not received any extension letters. The cases are being transferred to VSC but none of us have received any extension letters from VSC either. There is information about this at AILA
http://www.aila.org/RecentPostingList.aspx that says:
I-751 Receipting Update
USCIS Service Center Operations has informed AILA liaison that a processing problem at TSC resulted in delays in issuing I-751 receipts, which included biometrics notices coming prior to the I-751 receipts themselves, as well as biometrics notices and receipts bearing different file numbers. USCIS is working on the problem and expects resolution in the very near future. AILA Doc. No.l 08041668While it makes reference to correcting the problem in the near future, this problem has existed since early February and there is still no correction. They mention 'delay' but no one has received an extension letter yet, so it isn't delayed - it is not being sent. You will probably have to make an infopass appointment at your closest immigration office and get the I-551 stamp placed in your passport extending your green card status for a year. That is what the rest of us are doing.
You might want to check out this thread and add yourself to the list:-):
http://www.visajourney.com/forums/index.php?showtopic=118137