The Marine Corps put me through the 63 week Arabic course at the Defense Language Institute when I first joined several years ago. It was designed to take someone with zero proficiency in Arabic (but a good learning ability) and be a functioning speaker, reader, and writer of the language. Although it was very long, it worked. Since it had been a couple years since attending that course, a few months ago I began Rosetta Stone course offered through one of our education networks as a refresher.
It starts out at the basic level just like our DLI course, and progresses quite smoothly. As someone already skilled in the language, I was very impressed at how effective the teaching style is. If you have the cash to shell out for it, and have a serious drive to learn it, I recommend doing it.