For those who ever get Vodafone Roaming set up for Prepay I've just had some rather interesting dealings with Vodafone regarding this.

My girlfriend got prepay roaming set up however wanted to know why when she was in the UK and Australia she was being charged on her roaming credit card bill rather than it being deducted from her account (as auto roam should), particularly in the UK as autoroam also doesn't pay an SMS surcharge whereas on account does.

After some discussion it turns out she has been charged for calls in Australia and the UK on autoroam as well as being charged on the credit card roaming and the final result was a credit from Vodafone for this double charging.

I have no idea if this is a widespread or an isolated problem but it's certainly something worth checking on if you do have prepay roaming set up.