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The iPhone 4 antenna issue is really a two-part thing.
1) Covering the antenna with a fat, dense, fleshy material. (your hand) This can effect any phone, and is what Jobs wanted to talk about during the press conference. (remember the videos of competing phones dropping bars?)
2) Bridging the antenna gap. This is the issue that is specific to the iPhone 4. Apple didn't talk directly about this issue, but this is what the case fixes.
In either case, you may or may not notice the issue depending on how the signal is in your area.