  We present Advanced Camera for Surveys observations of MACSJ1149.5+2223, an
X-ray luminous galaxy cluster at z=0.544 discovered by the Massive Cluster
Survey. The data reveal at least seven multiply-imaged galaxies, three of which
we have confirmed spectroscopically. One of these is a spectacular face-on
spiral galaxy at z=1.491, the four images of which are gravitationally
magnified by ~8<mu<~23. We identify this as an L* (M_B=-20.7), disk-dominated
(B/T<~0.5) galaxy, forming stars at ~6Msol/yr. We use a robust sample of
multiply-imaged galaxies to constrain a parameterized model of the cluster mass
distribution. In addition to the main cluster dark matter halo and the bright
cluster galaxies, our best model includes three galaxy-group-sized halos. The
relative probability of this model is P(N_halo=4)/P(N_halo<4)>=10^12 where
N_halo is the number of cluster/group-scale halos. In terms of sheer number of
merging cluster/group-scale components, this is the most complex strong-lensing
cluster core studied to date. The total cluster mass and fraction of that mass
associated with substructures within R<=500kpc, are measured to be
M_tot=(6.7+/-0.4)x10^14Msol and f_sub=0.25+/-0.12 respectively. Our model also
rules out recent claims of a flat density profile at >~7sigma confidence, thus
highlighting the critical importance of spectroscopic redshifts of
multiply-imaged galaxies when modeling strong lensing clusters. Overall our
results attest to the efficiency of X-ray selection in finding the most
powerful cluster lenses, including complicated merging systems.
