I'm not sure because I got my iPhone at launch and I was just in to get another Airport Express, but I was looking at iMacs and I heard a conversation between a customer and an employee that went approximately as follows:
Customer: Do you have any iPhone 4s?
Employee: Only in black. 16 or 32?
Customer: 32
Employee: contract or sim free?
Customer: Vodafone contract
Employee: Yes we do, take a seat - I need to get some details from you...
That was when I left, but the guy seemed to walk straight in and ask the employee. I didn't hear him ask about a reservation, but then I didn't try and get one myself so I can't be sure if they were available. He might have already shown someone else his reservation and then been passed on or something. Still, it's worth trying if you are desperate to get one.