The Telegraph, from a memo written by Apple retail chief Angela Ahrendts:

Many of you have been getting questions asking if we will have the watch available in store on April 24 for walk-in purchases. As we announced last week, due to high global interest combined with our initial supply, we are only taking orders online right now. I’ll have more updates as we get closer to in-store availability, but we expect this to continue through the month of May. It has not been an easy decision, and I want to share with you the thinking behind it.

The whole memo is thoughtful and worth reading.