I'm always up for making the HTML easier for users on the site. Especially things that can be typed quickly with no knowledge of HTML. I have to remind friends often how to link to things. If it's possible to make a working shorthand that doesn't interfere with anything else, and without having hassle with an opt off/on, then I think the ability to shorthand usernames would be pretty nice for newer users.

Typing @mark is much faster than typing mark, even for me. Even <@mark> would be nice, as stated in previous comments, but you do run the risk of people never learning the actual mark up that way, too. It's in profiles, and in the FAQ, but I doubt I'd ever trouble myself to link in any other way if I could do it easier!

In addition, I do remember @ referring to both people and places ages ago, but I do like @usernames for referring to people now. And to me the @, being all curly - reminds me of DW's swirl. Mentally, I just make that connection.

I've used Twitter before, but I don't think of '@' as reminding me of Twitter. Dreamwidth has the ability to travel avenues that haven't been touched and come up with ideas that make the site easier. If you start adding a lot of periods and # into short hand usernames, they're not short anymore. Either it has to be super simple, or the same as it is, because the mark up is very easy on its own.