You're right. Did you catch this line in the Press Release:
"Since this mistake has been present since the original iPhone, this software update will also be available for the iPhone 3GS and iPhone 3G."
Well, if the software error has been present since the original iPhone, then why hasn't the "you're holding it wrong" problem also been there since the beginning?
Because the earlier models don't have the antenna built into the case like the new one. I think there's clearly a greater issue with reduced reception with the iPhone 4 because of it.
In retrospect, the "incorrect bars" doesn't sound so far-fetched to me. I either have "full bars" or nothing on my 3GS most of the time. I can't remember ever seeing a 3-bar signal since getting it 5 months ago.