The bumper doesn't fix the REAL problem
Apple better hope the solution lies in software, because the bumper isn't going to help. Yes it fixes the dropped signal issue - but the real problem for Apple is the growing perception that the company has produced a lemon.
Apple can continue to advertise the phone *as a phone* without bumpers - in which case they're possibly guilty of misleading advertising; or they can run adverts with the phone + bumper showing one of the f'ugliest things I've seen in a long time. Every time you see an iPhone + bumper you're going to think 'ah yes, that's the phone you can't talk on.' And Apple's rivals won't have to try too hard to find their advertising pitches.
If Apple thinks the 'solution' is to cover up their design it's a reasonable question to ask 'why do you pay Jonathan Ive?' There's little point in going to the expense of designing beautiful hardware if it only works when covered up.
The only long term solution is a fix that means the bumper is optional - even if you want to make telephone calls; rather than a necessary piece of the kit.
This problem is probably even more serious than Microsoft's 360 failures. Microsoft could at least claim the RRoDs only appeared after prolonged use and that it hadn't appeared in testing. If the WSJ's sources are correct, then Apple had been made aware of shortcomings in the design but chose to ship anyway.
The iPhone 4 - it's a great iPod but a terrible phone.