Apple's website says Applecare+ MUST be purchased at the same time as the iPhone, and their reps say that those who bought it without have either 14 or 30 days from date of purchase depending upon who you talk to.
However, the Applecare+ terms of service state:
(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.
http://images.apple.com/legal/applecare/docs/AppleCarePlus.pdf
Personally, I'd err on the side of caution and get it now if you're thinking about it. I've found that when dealing with a company representative about a warrantee where there are conflicting terms, they tend to take the path that's not the most consumer-friendly.