O2 is stating on their website that Pay and Go iPhone 3Gs will be available in a couple of weeks:
No word on if the white one will be available
Pay & Go customers can now enjoy the iPhone 3G without a monthly contract. The new iPhone 3G 8GB for Pay & Go will be available for £349.99 and the 16GB version for £399.99.
This also includes unlimited browsing and Wi-Fi for the first 12 months after you activate your iPhone 3G. At the end of the 12 months you can continue to receive unlimited browsing and Wi-Fi for just £10 per month. We'll notify you before the end of the 12 month period by text and you can easily unsubscribe if you choose to do so.
It will be available to buy from 16th September in O2 stores, The Carphone Warehouse and Apple stores.
Please note visual voicemail and call merging are not available with Pay & Go. You'll also need to change your data settings to use services such as Mobile Internet. Take a look at the activation tab on this page for more information.
No word on if the white one will be available