This is bad enough when you drop a call or can't make a phone call. But it is also affecting data service and this is the more likely time that you'll be holding the phone in such a way as to cause the problem to occur.
There's a video under Update 11 at Gizmodo that clearly shows it drastically affects data too:
http://gizmodo.com/5571171/iphone-4-loses-reception-when-you-hold-it-by-the-antenna-band
If you're not willing to put a case or bumper on this phone, then I think it's clear that you do NOT want to buy this phone right now.
There's a video under Update 11 at Gizmodo that clearly shows it drastically affects data too:
http://gizmodo.com/5571171/iphone-4-loses-reception-when-you-hold-it-by-the-antenna-band
If you're not willing to put a case or bumper on this phone, then I think it's clear that you do NOT want to buy this phone right now.