  (abridged) We present panoramic Spitzer MIPS 24um observations covering
9x9Mpc (25'x25') fields around two massive clusters, Cl0024+16 and MS0451-03,
at z=0.39 and z=0.55. Our observations cover a very wide range of environments
within these clusters, from high-density regions around the cores out to the
turn-around radius. Cross-correlating the mid-infrared catalogs with deep
optical and near-infrared imaging of these fields, we investigate the
optical/near-infrared colors of the mid-infrared sources. We find excesses of
mid-infrared sources with optical/near-infrared colors expected of cluster
members in the two clusters and test this selection using spectroscopically
confirmed 24um members. The much more significant excess is associated with
Cl0024+16, whereas MS0451-03 has comparatively few mid-infrared sources. The
mid-infrared galaxy population in Cl0024+16 appears to be associated with dusty
star-forming galaxies (typically redder than the general cluster population by
up to A_V~1-2 mags) rather than emission from dusty tori around active galactic
nuclei (AGN) in early-type hosts. The inferred total-infrared star-formation
rates in Cl0024+16 are typically >5x greater than those found from a similar
Halpha survey, indicating significant obscured activity in the cluster
population. We find evidence for strong evolution of the level of dust-obscured
star-formation in dense environments out to z=0.5, analogous to the rise in
fraction of optically-selected star-forming galaxies seen in clusters and the
field out to similar redshifts. However, there are clearly significant
cluster-to-cluster variations in the populations of mid-infrared sources,
probably reflecting differences in the intracluster media and recent dynamical
evolution of these systems.
