Any difference between O2 and CPWH contract?

Does anyone know if there are any differences between taking out an iPhone 3G contract with Carphone Warehouse rather than directly with O2?

If I go with CPWH will I be able to use the O2 account website, the normal O2 telephone support numbers, buy O2 bolt-ons etc?

I understand that some O2 contracts with CPWH are managed solely be them, you can't get upgrades directly from O2, the do the billing themselves etc..

Does this apply to iPhone contracts too?

Thanks for any help...

 

Great, that gives me a lot more options next Friday.