See http://bugs.python.org/issue3982 that's linked elsewhere in these comments for the kind of things that don't work on bytestrings
But really, I just don't like how http/ftp/similar protocols were designed in the first place. They're mixing binary and text in the same stream. That's ugly and when you have strict rules about text encoding in your language, the implementation also turns out ugly - what a surprise :)
Explicit is better than implicit.