For 8spd: Some companies make spacers that allow Shimano cogs to be used set to Campagnolo spacing. With this set up you should also use a Shimano chain.
For 9spd: Chris set his 9spd Campagnolo record bike up using 9spd Shimano Dura-Ace cogs and chain and it's been working flawlessly. There are also campy spaced spacers available to retrofit Shimano 9spd cogs.
For 10spd: Wheels manufacturing has released a campy compatible 10spd cogset made from a Shimano Ultegra 10spd stack. We are concerned that the cogs ride too close to the drive side spokes.