Emmm... the iPhone doesn't do 3G....
maybe a future version.
What I heard was that O2 have a contract with Apple for selling Apple products (O2 stores sell iMacs, ipods, etc), which is why O2 are the only ones that will be doing the iPhone, for the near future at least.
On the upside, with the iPhone being available officially hopefully the ebay prices will come down a bit!