I just would like to add that applying for OPT does not require biometrics (G-325A form). So technically speaking, there is not difference at all between having an A# or not.
Current Status: F-1 on OPT, expires in Aug 2011
04/24/2010: NL1 received
10/03/2010: AOS in Chicago
I don't think having a prior A# necessarily makes your process faster (especially during this early/preliminary stage). There is really nothing much to obtain if one has an A# based on OPT other than the fact that they have applied for OPT (which is basically a F1 'benefit') and their biographical info and some prior docs. If the A# were for prior I-485 filings based on some other criteria, then that would be another issue and hence, will in turn, probably take more time for them to locate/obtain the files from the holding office to see things in detail (which would happen at NBC). At lockbox, they will only look to see if there is already an A# assigned, if there is one, then they will assign a new one specifically for the purpose of this case (btw, I was assigned a new A#), and if there is not one, then they will create a new one. The only difference is it will most likely help save them some time to enter the G-325A info into their system which should not drastically delay your case in any way. I see your point about them having to enter all the info and feed the new docs into their system for the 'new ones' but I don't think that should be any reason for such length of delay.
And CONGRATS for getting the receipt no.! I am hoping that biometrics notice should start coming by the end of next week at most... Good Luck all the way to everyone!
Current Status: F-1 on OPT, expires in Aug 2011
04/24/2010: NL1 received
10/03/2010: AOS in Chicago