Hey,
Maybe, it will need to be unlocked (which can be very easily done by just Googling (I'm sure I can't fully explain it here)) and once you have your O2 sim inside it the unlimited data might work, but you will need to go into Settings --> General --> Network --> Cellular Data Network --> Then fill in the Cellular Data fields (and MMS fields if you want MMS too) with the O2 data settings below:
APN = mobile.o2.co.uk
Username = vertigo
Password = password
MMS
APN = wap.o2.co.uk
Username = vertigo
Password = password
I'm not completely sure whether it will work or not because this is how you normally set up unlimited data from O2 on normal devices or on iPhones on other networks, but according to O2 you need a specific iPhone tariff otherwise internet won't work. I'm not sure on this, because every network I've ever spoken to about an iPhone working (including T-Mobile) have told me it's impossible for whatever stupid reason (a lot of the time needing some kind of magical iPhone internet that only O2 can provide
) and I have 3G and MMS working on my iPhone 3G on T-Mobile, so I can't say right now whether O2 are lying and it won't work or not. What I would suggest is getting your hands on a friend's iPhone, trying your sim card in it, putting in the network settings above, giving it a reboot and seeing you can get internet through 3G or not.