Vodafone has a €3.00/week (500MB/week) option for the i-phone which is not bad.
At the same time Vodafone has the worst voice tariffs in Italy...
I suggest you Wind or TIM/MTV which have some EDGE coverage, which can be useful with an i-phone 2G. Wind has the Mega1500 offer for 8€/month while MTV (using TIM network) has a very interesting
1GB/month offer for just 5€/month.