http://www.psacard.com/services/psagradingstandards/
Lot's of information on the PSA site regarding their grading criteria:
Half-Point Grades:
Cards that exhibit high-end qualities within each particular grade, between PSA Good 2 and PSA Mint 9, may achieve a half-point increase. While PSA graders will evaluate all of the attributes possessed by a card in order to determine if the card may be eligible, there will be a clear focus on centering.
Generally speaking, a card must exhibit centering that is 5-10% better, at minimum, than the lowest % allowed within a particular grade. It is important to note that there may be cases where the overall strength of the card, such as the quality of the corners and print, will give the card the edge it needs despite the fact that it may exhibit only marginal centering for the grade. This is especially true for cards that find themselves within the bottom half of the PSA 1-10 scale.
Finally, keep in mind that qualifiers will not apply to grades that achieve the half-point increase since, by definition, these cards have to exhibit high-end qualities within the grade in order to warrant consideration. For example, there will not be cards graded PSA NM-MT-Plus 8.5 OC or PSA EX-MT-Plus 6.5 PD since the half-point is reserved for high-end cards within each grade.
At this time, only cards qualify for half-point grades. Coins, pins, tickets and packs will not receive half-point grades.