Once you receive your I485 receipt, you will receive the FP notice in a week or so. After you receive the notice, basically you'll have 2-3 weeks before you go to the ASC for your FP taken. If you miss the FP without informing ASC in advance, your I485 application is considered as being abandoned. If I were you, I'd not take any risk at this point to possibly miss such an important time-sensitive step. However if you do not receive the FP notice until Dec 15, 2011 and make sure that USCIS does not yet mail it to you, your leaving could be safe.
