I just bought my 2011 MBP yesterday and here's what the specialist told me regarding the student AppleCare. That yes, I can purchase the discounted Applecare on my 15" ($110 discount! beats B&H IMO) after purchasing my MBP. I can just come in, speak to someone and probably just need to show my student ID and I can get it.
Following up on purchasing AppleCare sooner or later, the specialist said that there IS a different level of "treatment". Not that they would treat you any worse, but that although it's not specifically written, if you have purchased AppleCare plan, you will be taken care of in regards to accidental spills or drops. But basically he said, if they (the genius bar/techs) see you already bought the full AppleCare, the Apple technicians will take care of you because you already went ahead and purchased the warranty plan. This is opposed to if you came in 6 months later under what he called "manufacturer's warranty" and something accidental happened, you probably would not be taken care of for being replaced on accidental coverage as opposed to someone that actually went ahead and bought AppleCare plan ahead of time.
This was his response as to why should I buy it sooner than later and he said those reasons listed above. He mentioned the store replacing cracked screens etc for people that bought AppleCare vs not doing so for people that did not already.
So I don't know! I do know, I really like my Tampa Apple Store and they've always been pretty straight shooters. Mind you, this was all information I got yesterday after purchasing my high-end system. He said he especially recommended I buy ASAP which confused me and thus he explained if I screwed something up myself, they'd take care of me rather than just using the manufacturer's one year warranty guidelines, which I would not be covered under accidental.