When you duplicate a Smart Object layer in Photoshop CS2, you're making an exact "clone" of the layer. So, if you update the Smart Object, all the copies change. This can be extremely useful, but sometimes you might also want to edit versions of the smart object a little differently. Here's how you do that:
After creating a Smart Object (through placing or "grouping into new smart object") choose New Smart Object via Copy. Rather than making an exact clone (like New Layer Via Copy does), this makes a copy of the smart object that still refers to the same smart object, but can be edited separately.
Try it yourself and you'll see where this can be very useful.