The spacing is NOT the same (see
www.sheldonbrown.com/k7.html#spacing), and it is not really close enough to make indexing work satisfactorily.
However several companies sell Shimano-spaced cassettes for Campy wheels, and Campy-spaced cassettes for Shimano wheels. Try AmericanClassic.com for those. There are also ways to replace the spacers in your cassette to make the spacing match, though I haven't done it personally.