To further add to Christina's post on this, I just
finished giving a presentation that discussed the
current growth charts.  Here's a snip from what I
presented.:

The NCHS growth charts are based on information
collected by the Fels Research Institute from 867
infants born between 1929 and 1975.  In that study,
the feeding method was known for 75% of the
infants - only 17% were exclusively breastfed, and
few were breastfed for more than 3 months.
Furthermore, the breastmilk substitutes used at that
time were less similar to human milk - higher
protein & fat, and the infants were more likely to be
given solid foods before 4 months.  Additionally,
measurements were taken at 3- rather than
1-month intervals, which obscured some of the
short-term variations in infant growth.  It is projected
that more than 5% of all infants, regardless of
feeding method, will fall below the 5th percentile in
any given month.  Because of this, there are a
number of investigators who suggest that the
NCHS charts are inappropriate for use with
breastfed infants.

Katherine Dewey, who has worked extensively with
studying growth parameters of breastfed infants
has found that breastfed infants who fell below the
5th percentile were much larger than would be
expected, averaging 31% for the six 1-month
intervals up to age 6 months and 52% for the six
1-month intervals between 6 and 12 months.
Although these percentages sound alarming, these
were healthy infants, fed on demand, and who
received solids at 4-6 months.  As far as activity
level, sleeping pattern, or morbidity, these infants
were no different from the breastfed infants with
faster growth rates.