Update: According to this Google+ post, pre-orders for the phone will begin later today. There's still no word on a shipping date.
Update #2: You can now press the "Add to cart" button. The phone is currently scheduled to leave Google's warehouse in 2-3 weeks:
One of the more interesting aspects of HTC's One M8 launch is the rapid way the company is releasing the phone, with major American and Canadian carriers getting access to the device in one day or less.