The USCIS has recently revised its policy regarding FBI name checks. If an AOS case is ready for completion with no other issues outstanding other than the FBI name check, the USCIS will complete processing of the case if the name check has been pending for 180 days or more. If the AOS is approved and the USCIS later learns that the FBI name check came back with a negative determination, the USCIS will revoke the AOS approval. Please see the press release from USCIS on this issue:
http://www.ilw.com/immigdaily/news/2008,0229-FBIcheck.pdf.
Also please see the following link for additional information on the USCIS' plan for working through AOS cases impacted by the change in policy:
http://www.uscis.gov/files/article/NameCheck_2Apr08.pdf.