The boundaries between present-day people and non-people can be sharper, by a fiat of many intervening class members being nonexistent, than the ideal categories. In other words, except for chimpanzees, cryonics patients, Terry Schiavo, and babies who are exactly 1 year and 2 months and 5 days old, there isn't much that's ambiguous between person and non-person.

More to the point, a CEV-based AI has a potentially different definition of 'sentient being' and 'the class I am to extrapolate'. Theoretically you could be given the latter definition by pointing and not worry too much about boundary cases, and let it work out the former class by itself - if you were sure that the FAI would arrive at the correct answer without creating any sentients along the way!