Let's just say that I don't just teach this stuff. I also run a web design/consulting business in which I design the layout of websites, program the back end (databases, PHP, ASP) and occasionally get called in to do some work in Flash with ActionScripting.
To be truly successful at this you have to know a lot of different areas, including knowing what you don't know. With those areas, it is good to team up with someone who can help you out. I often work with designers who don't know as much as I do about coding, and likewise, I seek their advise, input, and sometimes work on design aspects when necessary.