  We determine the abundance of i'-band drop-outs in the recently-released
HST/ACS Hubble Ultra Deep Field (UDF). Since the majority of these sources are
likely to be z~6 galaxies whose flux decrement between the F775W i'-band and
F850LP z'-band arises from Lyman-alpha absorption, the number of detected
candidates provides a valuable upper limit to the unextincted star formation
rate at this redshift. We demonstrate that the increased depth of UDF enables
us to reach an 8-sigma limiting magnitude of z'(AB)=28.5 (equivalent to
1.5/h{70}^2 M_sun/yr at z=6, or 0.1 L*(UV) for the z~3 U-drop population),
permitting us to address earlier ambiguities arising from the unobserved form
of the luminosity function. We identify 54 galaxies (and only one star) at
z'(AB)<28.5 with (i'-z')>1.3 over the deepest 11arcmin^2 portion of the UDF
field. The characteristic luminosity (L*) is consistent with values observed at
z~3. The faint end slope (alpha) is less well constrained, but is consistent
with only modest evolution. The main change appears to be in the number density
(Phi*). Specifically, and regardless of possible contamination from cool stars
and lower redshift sources, the UDF data support our previous result that the
star formation rate at z~6 was at least x6 LESS than at z~3 (Stanway, Bunker &
McMahon 2003). This declining comoving star formation rate (0.005
h{70}M_sun/yr/Mpc^3 at z~6 for a Salpeter IMF) poses an interesting challenge
for models which suggest that L>0.1L* star forming galaxies at z~6 reionized
the universe. The
  short-fall in ionizing photons might be alleviated by galaxies
  fainter than our limit, or a radically different IMF.
  Alternatively, the bulk of reionization might have occurred at z>>6.
