What's the likelihood of this selling out on launch day?

Depends on how much stock they get. If they undershoot, it could very well happen, as this is hands-down the most widely anticipated device in our young platform's history. The Lumia devices are all selling well here in Canada, according to Canadian tech blog mobilesyrup.com, so if you NEED this device right away, I recommend preordering. Even the 710 is selling out up here.
 
Don't think of it as a dissappointment if it doesn't. You may see some larger cities run out of stock in some stores, simply because you can't get this anywhere else other than AT&T Stores (I don't think BestBuy will have it on launch day, Costco will not). I would look for news on the pre-orders, because if they sell a lot of pre-orders, you may see a lot of stores without any on launch day.
 
From talking with the sales manager of my ATT store here, I think there will be pretty good demand for it and the sales staff here seem to really like the device and they will be trained on it on the 30th. That being said I think they will have sufficient stock to seel a bunch and they have more than enough available. Plus they will be selling the white in about three weeks and hopefully have the ability to get more black and cyan if need be from the warehouse(s)
 
Not all stores are closed that day. All the mall stores are open because they have to abide by mall hours. All I know is my store is closed :D
 
I have a few friends that work for att, they seem to think they will sell out the 9th, they have all said they are going to push them hard. So many customer complaints from the androids, never knowing how to use them ect... But they said they do not have many in store. I was gunna wait for the white, but think I am gunna end up getting the blue