If you go into a Virgin Shop and ask them to cut your SIM down to a micro SIM size chances are they wont as they will not have the tool to do this.
You can actually phone up Virgin on 789 and get a free Micro SIM as your a current customer. They will send you out a 'replacement SIM' which will be Micro and all ready to pop into your iPhone 4.
If you don't want to go down the road of ordering a Micro SIM (free) from Virgin then your other option would be to buy a Micro SIM cutter from ebay (around about £3 to £8) Though with Virgin's free Micro SIM replacement thing in place I don't see why anyone would want to cut their own anymore.
Also, because your a Blackberry user your going to have to call up Virgin and ask them to change your Blackberry Data (services) to 'normal' data. This is because BB data is different to regular data. If you don't do this then the 3G (cellular data) will not work on the iPhone.
Hope this helps
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