This is my understanding, but correct me if I am wrong.
Approval of 1140 does not mean that the immigrant visa number as been assigned to you. Once your 1140 is approved, USCIS forwards that information to the state department, who then assigns the visa number, provided the quota is available. If the priority date is not current, the visa number will not be allotted till it becomes current. You can apply for your I485 only is the visa number has been allotted. USCIS rejects I485 applications if it is sent before the date on which the visa number was allotted.
This raises a question, what about folks who filed their I485 when the priority date was current? Most likely they would have gotten a visa number. So their 485 is eligible for adjudication. But I don't know how USCIS is going to treat such cases, though I feel they could be adjudicated as the visa number has been assigned. We need to find out how such cases were treated in March/April of 2000 when priority dates retrogressed the last time around.
The major goal of USCIS in 2000 was to move towards making priority dates current. (To achieve this, they implemented a change where unused visa quota could be transferred between countries – my memory is a bit fuzzy on this issue). Now if the dates retrogress, then they seem to be actually negating what they tried to achieve.
The funny thing here is, the faster USCIS adjudicates, the faster the visa numbers gets used up, and more likely the priority dates would retrogress!
Just my thoughts.......