They clearly knew about the antenna issue before the phone was released and I'm sure they could have delayed it, but financially that wouldn't have been the most popular of decisions. They also probably underestimated just how much the antenna issue was going to be picked up on.
I think for Apple now, this is probably the best thing they can do without having to do a full recall. If the issue is fixed, they restore confidence in people to buy the phone... I personally have held off getting one. Wouldn't surprise me if they release the white phone at the sametime that they announce they've fixed the problem.
Presumably they might offer a replacement to people if they want one. The benefit of that is that compared to an earlier full recall, not everybody will take up the offer. I'm sure the returned stock could be potentially corrected to the hardware fix and used as their refurbished units for exchanges/sales, or at least have used parts available from them.
So ultimately, it would be like a recall, but wouldn't cost them as much.