I've noticed this problem, as of now, only with a generated city map.
When a Creeper explodes -- stuff appears from blocks it WOULD have destroyed, but the blocks themselves do not actually disappear. The little bits still fall through the floor, sometimes, but the blocks are definitely 100% still there.
Update --> Creepers work just fine on SP saved maps that have been made by the random generator.
--> tommerry has claimed building/destroying makes it work, but this hasn't made a generated city map work. (for me.)
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"I want to see forum posts saying "how do i kill dragon" with replies reading "lol"."
---> vede claimed Notch said this (and it is awesome).