Look in the paperwork that they gave you when you got your ibook back. When I paid the flat fee, Apple guaranteed it for 90 days, and usually they can fudge that a bit. My MLB failed for the 2nd time after a little less than 4 months, and they took it back and fixed it for free.
If all else fails seriously throw a fit. They have precedent of taking in repeat repairs and they should take yours back if you talk to the right people.