O2 has opened pre-orders for the 3G iPhone for existing iPhone customers, via texts and emails sent out this morning to those who'd registered an interest in the device.

Pre-orders were supposed to open from 8am this morning, with handsets due to be couriered direct to customers this very Friday.

To recap, it involves signing a new 18-month contract, with the 8GB model being free on O2's £45 and £75 monthly tariffs, and £99 on the £30 and £35 tariffs.

However, the bad news is that O2's online shop appears to have flubbered to a standstill under the rush. Currently, clicking through takes you to a page saying, "Our upgrade shop is currently unavailable. For now, take a look at our iPhone tariffs".

With a one-iPhone-per-person policy, and a first-come-first-served approach, it could be a busy morning for O2's web techys.

UPDATE: the upgrade section of O2's site is still down at 12.30pm, but the operator's press office has put out a message via its official Twitter feed: "We're trying our best to get the O2 site up and running properly. We'll update you when we get more news..."