I believe there is some reason for concern for preterm infants.  Please
see the article:
Transmission of cytomegalovirus to preterm infants through breast milk
by: Vochem,Hamprecht,Jahn, and Speer found in The Pediatric Infectious
Disease Journal Vol. 17,No. 1, January,1998.  It concluded that "There
is evidence that the most immature infants are at the greatest risk to
acquire an early and symptomatic CMV infection".  (Italic's mine)  67
preterm infants were studied.  27 women were CMV-seronegative @ birth
and breastmilk and infant's urine remained neg.
29 women were CMV IgG-seropositive with 23 of 27 of these mothers
excreeting CMV in their milk.  CMV infections occurred in 17 of the 67
with the 17 coming from seropositive mothers who excreted CMV and
breastfed.
Janet Bryant RN,IBCLC