It's pretty much hit and miss for anyone trying to order an iPhone 4 online. Mostly, it's misses as the server connection would time out.
So the best way is likely to go to Radio Shack if you're fortunate enough to live or work near one. And if you don't but really want an iPhone 4 on launch day, drive there.
I decided to go there after having issues with both Apple and ATT's online ordering. It's just brutal. Then around 10AM PST, I drove to the strip mall where there was an ATT store as well as a Radio Shack.
The ATT store had a long long line. I was in despair. So I decided to see if Radio Shack was accepting pre-orders. Before I left for the little iPhone pre-order trip, some folks were saying Radio Shack would not be part of the retail stores accepting pre-order. Maybe this is why there was no line there.
When I walked in there was one guy in front of me. He had just came from being the second to the last person at the ATT store. Now it was first at Radio Shack. And Radio Shack was accepting pre-orders.
What's also cool was that the folks there were very knowledgeable and had me all set to go. I left before they were able to get through the pre-order process but the guy who helped me called me an hour later telling me that my order went through and that they would call me to set up an appointment for activation.
So give Radio Shack a go.