Hi, I have entered the USA and exited from a different port many times, without any problems. Now that the ESTA system is properly up and running, you do not have to fill out the old green visa waiver form, just the blue and white customs form.
The ESTA is pretty easy to fill out on-line, you now have to pay (I think it is $15?) but you will need your passport details, and if you have to address (inc ZIP code) of your first nights stay, it will help, and save you having to get back into the system to enter these details. Once the ESTA approval comes through, it is valid for 2 years.
Have a great trip.