I like the idea of a shorter way to indicate an off-site user. But I don't like the Twitter markup at all. It's one of the most confusing things I find about the twitter service. My main problem with it is that @ actually means something. It's not just a symbol that we can use to slot in for something else.

Or, let me try to explain this better since I think there is an argument for @username to maintain the meaning of the @ symbol. But to me @ is a locational symbol. The username comes before it like in an e-mail address and it maintains a link to grammar - "I'm meeting username at dreamwidth(.com). Twitter has started using @ as a directional symbol. As is "I'm talking @username." And that doesn't work in my mind.

I would definitely want to either turn it off or still be able to use the html while it's on.