Twitter vs. Email. Fight!
codeulate.com
codeulate.com
Maybe I'm old school, but for me, there actually is something appealing about expressing a thought in more than 140 characters and specifically to a person of my choosing. I'm Web 1.0 like that.
So different, that we created posterous to bridge the gap. =)
foulmouthboy is right: There may be some overlap between these two communication mediums but they have two different purposes.
However, email is the incumbent technology so it takes a major paradigm shift to unseat it. Something I haven't seen any new technology come close to.