Yea, Zedbrush could probably remain 32bit even 10 years from now, when most systems are running a 128bit architecture (or whatever the future bit technology will be).  The wonders of volumetric sculpting.  Once you convert the dynamic voxel-based model to hard polygons, however, the impact on memory consuption is the equivalent to dropping an anchor into a bathtub.  It could easily handle 1 billion voxel faces, but ever try to import 1 billion quads into zedbrush?  The limit of it's ZRemesher is about 8 million polys (tri or quad), limited by the 32bit, 4gig architecture.  Not that this is ever a realistic roadblock to most projects, but it's there.

Zedbrush 5 will no doubt be 64bit.