Al,
Let's see if I can provide a useful summary. I'm not an immigration lawyer, or a representative of any fight schools (JAA or FAA), but I do have experience of dealing with the INS.
The first thing to remember is that the INS is a poorly resourced beaurocracy, staffed by many people who don't understand the rules properly themselves. There is always a risk that the US immigration officer meeting you at your port of entry might be having a bad day, feel ultra-cautious, or just dislike your face or clothing. [In my personal experience they are usually either very courteous and professional, or just going through the motions and don't really listen. In either case I've never had a problem.]
So you've got two options:
Get an M1 visa. This probably isn't legally necessary since PPL training falls into a grey area, but it is watertight and you shouldn't have any problems with US immigrations. The disadvantage is you have to apply for & pay for a visa, and you are then stuck with one school.
Use the visa waiver. The advantage is that it's simple and since you are entering as a tourist, you can change flight schools as often as you want. The big disadvantage is that US immigration are more likely to deny you entry, especially with the current paranoia about foreign flight students. Bluntly, if you do not look like a white Anglo-Saxon, then don't even consider this route.
So, get an M1 if you want to be safe & don't mind the restrictions, and use visa waiver if you want the flexibility and are prepared to take the risk of being denied entry.
The USA is a great place to fly, and I wish you luck with your flight training.