I actually have like 6 twitter accounts (I'm running out of email addresses to register with!) My main one is calevans (prefix it with twitter.com/) and there I just usually chitchat with friends, post things I find, try to be witty. Since I usually only blog once a week, I also post links to my blog posts but I try to keep that to a minimum. It does however double the traffic on any given post vs. non tweeted posts. That is due in large part to the fact that I don't get that much traffic on my blog anyhow.
The other twitter accounts are project specific, my blog doesn't have one but 2 of my podcasts do and the conference that I chair does. I publicize these for what they are, announcement channels. That way I don't turn people off with my channel just to pimp my projects. I've found that those who want to use twitter as a notification channel, readily subscribe to the other channels. The ones who don't are glad that I set them up.
The only problem with this scheme is that I keep having to log out/log in to announce different projects. I usually keep Safari (I'm on Windows, it's not my primary browser) logged out of any twitter account and when I'm not working with my main account, I post from Safari and then try to remember to log back out.
All-in-all, twitter, if used right, is a great marketing tool. As with any tool, you can poke yourself in the eye with it. I've got another idea about twitter but I'm working on a blog post about it.
p.s. yes, that only accounts for 4, I've abandoned one because the name was too long and the other was a typo when I registered it.