If Jakob Nielsen is writing a usability article, we know Twitter is now 100% mainstream. Interestingly, this article though only covers release of corporate news information with tips for both writing punchy copy and for timing (9:01 is better than 9:00).
A little chillingly though, he indicates that Twitter “decay” (when people stop clicking) is fast. Twitter updates so quickly that messages are very quickly lost in the shuffle. As Nielsen comments “Once [the messages] scroll off the first screen, they’re essentially 6 feet under.” As a result, he still recommends e-mail for messages with a relatively long life span so that users can sort through their “pile” at a later date. No argument from me.