It seems that raster catalogs are not supported (yet), here are some ideas:
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If the mxd contains a raster catalog, at least give a warning, and maybe the option to export the individual rasters to ArcPad
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Native support by ArcPad for rastercatalogs, either in a .dbf, or in the .axf.
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Having the rasters inside the backgroundlayers.axf would also be nice, but I'm not sure how this will affect performance.