I don't think long lines of people who reserved ipads and are having them set up will be a problem.
My theory is that the people who are excited enough by the ipad to reserve one without ever seeing it in person, will not be the type of people that will need someone to help them set up email and apps. The people who reserved theirs on April 3 are the hardcore Apple people who already most likely own an iphone, mac, ipod or some combination of the three. They will know how to set one up and probably not want to wait around a crowded store to be shown how to set up email, which is extrememly easy anyway. I expect most to be in and out, and itching to get their ipad home to so they can set it up on their own mac and start playing with it right away.
There still might be a couple people there on opening day that would want help setting it up, I'd say its a very small percentage.