With the announcement that customers can now pre-order their "upgrade" to the iPhone 3G, O2's website appears to be seriously struggling with demand.

Despite O2 sending a mass email to inform wannabe owners of the coming soon device that pre-orders are now being taken, the site is "currently unavailable".

One disappointed Pocket-lint reader says: "Completely impossible to get onto the O2 upgrade site. I called O2 and was told the site has frozen due to high demand and has been taken offline for 'maintenance'".

O2 has warned of high demand for the new phone: "We do, however, expect demand for iPhone 3G to be very high so orders will be processed on a first come, first served basis so that we can be fair to all customers. Orders will be limited to one iPhone 3G per customer".