Shimano Crank on Campy Square Taper BB
 

I know that Shimano and Campy square taper bottom brackets have different tapers, but any problems in practice from mounting a Shimano crank on a Campy square taper bottom bracket?

You risk ruining a part for a $20 Shimano cart bb. The chainline will be -4.5mm compared to a JIS/Shimano bb of the same length as the campy.

http://www.sheldonbrown.com/bbsize.html#square
http://www.sheldonbrown.com/bbtaper.html

The Campy (ISO) taper starts out slightly smaller than the Shimano (JIS) taper so a Shimano crank will go further up the spindle when torqued to spec. As long as it doesn't go so far as to get to the round part of the spindle or have the fixing bolt bottom out on the spindle end you should be ok

+1 hillrider