I suspect that Apple is working on a contingency plan if antennagate doesn't blow over by the time the Free Bumper offer expires at the end of September.
This contingency would involve laminating the stainless steel band (or using some kind of polyimide varnish). However, since this is a significant change to the production process, it's not going to be ready until October. So they couldn't announce it at Friday's press conference, because it would impact short term sales of the current model.
Come October, if I'm right, they could offer a solution whereby current iPhone 4 owners can bring their phone in to an Apple Store to get its steel band laminated (or varnished) free of charge - although this process will inevitably require the phone to be off site for a few days, which will mitigate the number of users who take Apple up on the offer.
The delay of a couple of months will also give Apple time to evaluate if the media storm blows over. If, by mid-September, users have come to realize that this really isn't a major issue, and sales remain buoyant, they'll mothball their lamination/varnishing plans.
The above is only speculation, but it's much more plausible than Steve Jobs's "Eminem bandaid" theory ;)