If you’re in a strong signal area, you may not ever see the
effect, because even with the attenuation from holding the phone
you’ll still have plenty of signal left over. But that
doesn’t mean you aren’t going to be affected by the issue
unless you are never, ever in a weaker signal area — and the
second test above suggests that 3G data transfer rates are still
going to be slower anyway.
His conclusions: with a strong signal, iPhone 4 reception is slightly worse when “holding it wrong”, but you’re unlikely to notice. But with a marginal signal, how you hold the iPhone 4 matters very much.