In previous versions of Illustrator, selecting two or more objects and performing a Pathfinder operation (Add, Subtract, Intersect, etc.) would result in what’s called a Compound Path. The original shapes remained, but only the results of the Pathfinder operation would be visible. The Compound Path approach gave you the ability to “reclaim” the original shapes if you needed to.
Above: Pathfinder creating a Compound Path after a Subtract operation;
note the “leftover” original shapes. They’re not visible,
but they’re still accessible in Outline mode.
But most users wanted the “pure” finished shape: holding down Option or Alt while clicking a Pathfinder option would eliminate the leftovers without making a Compound Path:
Above: Prior to Illustrator CS4, holding down Option (Mac) or Alt (PC)
while choosing a Pathfinder operation would give you an expanded result,
with the non-visible object fragments permanently deleted.
Neat, but what if you ever wanted the old stuff back?
If you’re in the habit of holding down Option/Alt while performing a Pathfinder operation, you might want to change your habit when you upgrade to Illustrator CS4. Now, Pathfinder results are automatically expanded, with no modifier key necessary, and Option/Alt now has the opposite meaning: it now prevents expanding, and results in the creation of a compound path. So, if you want just the resulting shapes (with no leftovers), don’t press Option/Alt while performing a Pathfinder operation.