i got my acceptance via fed-ex on st. patty's day (application went in on 1/15 on the nose). the letter said they still needed information from me, even though i thought it had already been taken care of and verified. either way, to answer my own question earlier, acceptance packages must be turned in within two weeks of receiving the letter.
@chonofish- my aunt, who is in admissions at columbia, told me it's best to contact them (admissions) and ask for the specific reasons why you were deferred so you can work on that for the next application period. in all cases i think you get the best responses if you do a walk-in. my undergrad was spotty, but had a 4.0 in all my science pre-reqs. did well on my net exam. and had some good reccs. how was your essay?
hope that helps:)
and ps- proof of citizenship was listed as missing on my application as well!