On June 19th the iPhone 3G S will be available from both Apple and AT&T stores at 8am and 7am respectively.
AT&T has also released a video (above) on youtube clarifying how things are going to pan out on iPhone day. Right now you can pre-order the iPhone online for delivery or at a company owned AT&T store to pick up on June 19th.
There will two lines at AT&T stores on June 19th, one for those who have pre-ordered and one for those who haven't. Once the doors open those that have pre-ordered will have priority over those who haven't.
If you want the best chance of getting your iPhone 3G S on June 19th be sure to pre-order, and then be in the front of the pre-order line on June 19th. If history is any indication iPhones will be in short supply for the month or so after initial release, and stores will be stocked with limited quantity.
edit: to be able to pick it up in store on June 19th you must pre-order in person at the AT&T store you plan on visiting.
Prices are as we already know for new customers or those who qualify for upgrades. $199 for the 16GB model and $299 for the 32GB model.