I think Winnie, that you raise valid questions. Here's my (personal, not professional, lol) answer:
I believe, from all the reading I've done here, on other sites, in books, etc. that autism is both a disease and a developmental disorder.
I believe, that children with autism have a genetic, metabolic condition that makes their bodies unable to perform certain crucial functions, such as detoxifying metals for example, without intervention. This metabolic disorder is expressed, nost noticeably, through the familiar behavioral and cognitive symptoms of autism. Their bodies and their brains are not working properly (disease), and as a result, their development is compromised (developmental disorder). Untreated, they can sustain permanent damage I would imagine, depending on how severely affected they were (by both the metabolic disorder, and exposure to environmental toxins).
I believe there is sufficient data from lab reports and whatnot of various ASD kids to demonstrate that there are also PHYSICAL signs of ASD (which are less obvious than the behavioral ones).
Now, treating the metabolic disorder by supplementing the missing body chemicals (enzymes, neurotransmitters, etc.) will treat current symptoms. It may undo a certain amount of "damage", by clearing some toxins, and providing the body with the tools to heal injured tissues and systems. It won't, however, change the child's genetic makeup. Vitamins, no matter the method of delivery, are not gene therapy. Gene therapy is still extremely new, very controversial, and very complicated. Autism is a LONG way off from being able to be addressed by gene therapy, as we don't even understand yet WHICH genes cause this metabolic disorder (and it's likely there are a combination of genes).
In this way, I think Rick's diabetes analogy was good. Children with diabetes are unable to produce insulin sufficiently. As a result, they get elevated blood sugars, something you cannot see without blood tests. What you DO see are outward signs & symptoms, such as (for example, since it varies from child to child) headache, confusion, blurred vision, trembling hands, irritability. The high blood sugar, and resultant symptoms, can be addressed through managing the diet (restricting sugars) and/or providing the body with insulin via insulin injections. Both eliminate the symptoms. Neither cures the disease. Without continuing treatment, the symptoms will return, because their is no way, at this time, to treat the cause, the body's inability to produce insulin.
Another example is kidney failure/disease. Affected people, awaiting transplant (the "cure"), have to intermittently go for dialysis, an external process that does the kidney's job of ridding the body of toxins. One dialysis session doesn't fix the problem. The body continues to make toxins, and the person continues to be exposed to toxins from the environment (food), which need to be eliminated from the body.
Autism is the same way, IMO. You can't "fix" the underlying cause, and you cannot, realistically, avoid "recontamination". The world is so full of toxins (that are harmless to those with intact elimination/detoxification systems), you cannot possibly shield your child from them all. Furthermore, many of these "toxins" are our body's own byproducts, and completly unavoidable.
Until we have clearly isolated the cause, and found a way to reverse it, there will be no cure for autism - we can only treat the symptoms (either at the external, behavioral stage, or the internal, physical stage), and treatment will have to be ongoing.
Last but not least, I'm glad that Rick mentioned that children "recovered" through biomed will still have delays, as nutritional/vitamin/etc. therapy cannot magically "undo" the missed learning. It can make the body READY for learning, but the work still needs to be done!