I surprisingly got an order through on the Apple Store pretty late in the day (around 7 pm PST, though I did make three tries and wasn't sure if any went through until I called Amex to verify that one of the charges had been authorized -- I got my confirmation email from Apple about 3 hours later)
But all that to say, I have the same exact concerns about activation when I get my phone delivered on the 24th. If it was such a mess with AT&T's servers today for the pre-sale, won't it be just as bad when activation time rolls around? I mean I'd guess less volume since not all the traffic today ended up in iPhone sales... but not very confidence building.