http://www.uscis.gov/graphics/lawsregs/handbook/AC21Intrm122705.pdf
下面的说法比较混淆. 和9月分的memo有些矛盾.
Question 11. When is an I-140 no longer valid for porting purposes?
Answer: An I-140 is no longer valid for porting purposes when:
A. an I-140 is withdrawn before the alien’s I-485 has been pending 180 days, or
B. an I-140 is denied or revoked at any time except when it is revoked based on a withdrawal
that was ted after an I-485 has been pending for 180 days.