The 1st gen iPhone does access VFN's GPRS network from time to time in order to update weather, stocks etc. So even if you don't use safari or check your mail, you will still get a data charge.
Not sure how much it would use in a day though.
There are 2 ways of stopping this service.
1) Contact Vodafone and turn off GPRS for your SIM
2) Install BossPrefs on your phone and turn off EDGE (VFN does not have an EDGE network but the setting works for GPRS too).
There is a pretty comprehensive discussion on BossPrefs at the Kiwi-ise forum.
Hope this helps.
JavaJunkie: Sorry, I forgot to mention that you CANNOT turn off GPRS using the iPhone preferences alone.
Prior to firmware 1.1.4, you could just put a random APN in the EDGE setting and disable GPRS. But this no longer works. As far as I am aware, you have to install BossPrefs to override the GPRS settings.
brownbox: i read somewhere that the iphone 1g slowly uses data and this must be disable either from vodafone or from the iphone settings. Im wondering, how much does it actually use? Or is this just a myth?