Jon's the best one to answer this question, but as for the basics, you don't need to be a high bandwidth subscriber to post lessons.
To become an instructor you'll need to post at least 2 lessons. (preferably more) There's a lot of criteria to cover but obviously excellent guitar technique and knowledge of the instrument, an ability to write out accurate descriptions and tabs, basic theory and using proper grammar and spelling. The ability to record clean and accurate MP3 recordings. As far as I know there's a new requirement for video posting because the site is quickly updating it's video lesson content. So that would mean a video camera, some basic editing equipment and a way to convert your finished movie into flash format. An ability to cover subjects that haven't been covered previously would be a big help too. I think I heard mention of strumming techniques, tutorials on alternate tuning, more beginner lessons....
Best thing to do is spend some time on the site and get an idea what the instructors are already posting. If you can do as good, or better and can cover some topics and styles not already covered by several instructors here, then there's a good chance you'll get accepted.