I bought my iPhone online and used my PAC code from Orange at the time of ordering.

Today, Vodafone sent a text to my old phone saying my number would be transferred on Monday. However, I would like to activate my iPhone now and set it up ready.

Will it be ok to activate it before my current number is transferred on Monday? Would like to get used to it over the weekend

I believe so. I bought an iPhone yesterday, and am still waiting for a PAC code from BT (I believe it all comes through Vodafone), but have set it up and played with it. It just uses a different phone number for now.

I was in the same situation , I activated my iPhone 4 before vodafone ported my number over from O2.

Just to warn you (when transferring numbers) that my iphone 4 could make telephone calls but I could not recieve calls for a further 24hours - may not happen in every case but just a tip as I was not told this at the time.