I have been skimming this thread since the inception - but haven't read it in full so what I say might have been covered...
My involvement in bonsai has spanned about 10 years, but I can honestly say the past 2 years have been the only worthwhile time. In those two years I have spent quite a bit of time with Walter, some good time with Hagedorn, and a little time with BSOP, Brent/Dr. Bob, Gustafson, Boon and some of his students (not all at the same time). I have spent a lot of time on Internet forums, have been very involved behind the scenes with Oregon Bonsai, and have had informal conversations with several American students currently apprenticing in Japan. Does that make me an expert or a voice for this topic? Nope...
But, what I see is that America is still developing in the art of bonsai. Here in Portland we have three facets to bonsai - old club members that enjoy going to meetings and playing with little trees and serious upcoming hobbyists and artists (split into two groups). The old timers have mostly been participating in bonsai for a number of years, and are very adept at creating bonsai that look like those in the books of Naka, Gustafson or Tomlinson - for good, bad or indifferent. I see nothing really creative nor original, but they do look like bonsai.
The second group splits into two facets. First, one led by the teaching of Boon and locals such as Scott Elser - and of course the recent addition to the area of Hagedorn - are doing two things. They are taking the art to the highest level & they are heavily inspired by Japanese teachings (though they do have excellent native stock, many are looking at adding Japanese foliage to this native stock). The second group is made up of of individuals that are looking to take American trees to their fullest potential (and keeping these trees native) - working from the teachings of Walter, Boon, Hagedorn and others - and work towards the ultimate in the "naturalistic" style preached by Walter (but excelled upon as well by Boon & Hagedorn in their own ways). We in this group are young, still learning, and are still developing our own niche in this style.
So, today in the Portland area I see 3 distict American styles being practiced - which I think is what has kind of been stated through this thread. That said, I do believe that we are on the verge of creating a dominant style in this area (and hopefully the rest of the country) and the next 5 years will be telling. In all honesty, I think that when the two apprentices return from Japan in 2 & 4 years, it is hoped that one resides in Portland while the other will for sure -this new style will begin to develop. I believe that the material that Oregon Bonsai provides will be combined with the naturalistic style and will receive a tweaking of Japanese influence - I believe this will combine all of these into a sort of melting pot to create and define a style that in time I believe can become the major American Style (which is basically what one group is doing here in Portland right now).