All I found was this:
http://handleopenurl.com/scheme/twitter
Which is referencing atebits and Tweetie2 rather than Twitter for iPhone. The number of things I can do according to that list is very limited. No way to follow someone. I don’t think Twitter for iPhone is that limited given the fact it has been bought by Twitter.
An URL scheme on the iPhone is to “talk” between apps, for example, with the tel URL scheme you can dial a number, with the mailto one you can open Mail with a compose window ready to send an email.
An URL scheme is NOT an API, if you want your app to ask the user if they want to follow you (for example), you should use the Twitter API, like the one that comes integrated with iOS 5, for example.