I think there will be lines but not to the extent of the 3G. Pre-orders and the magnitude of changes (not as much changed between the 3G and 3Gs then 2G and 3G) will mean a smaller lines. I don't know if they did this for the 3G but both ATT and apple are saying they'll open early (where allowed) to facilitate the sales.
I think people who are upgrading from a 2G or a 3G are mostly doing it online but those who are interested in getting their first iPhone will probably want to go in and do it in person in case there's a problem with activating, or switching their phone number