Yes, you can purchase it after you buy the phone.
OK, I actually read the Applecare+ terms and conditions.
The Apple web site for Applecare+ "Important note" says they have to be purchased together. However, it is interesting that it doesn't seem to say this in the actual terms and conditions that it has to be purchased together. In section 4.2 it says coverage does not apply to:
(viii) A pre-existing condition on any Covered Equipment if you purchased the Plan after you
purchased the Covered Equipment
Then, later in section 11 it says:
(ix) The terms of the Plan, including the original sales receipt of the Plan and the Plan Confirmation,
prevail over any conflicting, additional, or other terms of any purchase order or other document, and
constitute your and Apples entire understanding with respect to the Plan.
(x) You must purchase the Plan while your original iPhone is within Apples One Year Limited warranty.
Apple is not obligated to renew this Plan. If Apple does offer a renewal, it will determine the price and
terms
So, according to their terms and conditions, you can purchase it within one year. But, it won't cover anything that happens before you purchase it. So, if you drop and smash your phone in 6 months, and then try to buy the warranty and have it fixed, that shouldn't be covered. But, how would they know it broke before you bought the warranty? I mean, it would seem fairly obvious if you bought Applecare+ and a few days later walk in with a smashed phone that "just happened" and want it replaced.
But, I think for all of us doing pre-orders, it should be fine to buy it separately from Apple if the carrier does not offer it at check out. To be safe, I would buy it right away though.