I think starting with a 600cc bike is fine as long as you ride smart. I started with a 95' Suzuki GSX 600 3 years ago and still have it today. It wasn't too powerful to learn with yet powerful enough to keep me happy when I got better at riding. I wouldn't go out and drop $5 grand on a newer bike to learn with. More than likely you are going to lay it over at least once. Why waste money on something really nice when you are just going to damage it (cosmetically and mechanically)?

By the way, I'm selling my bike now if anyone is interested. Just got a newer one.