Let me first state that, as a publisher and a user, I love Twitter. I love Twitter's portability - whether it exists between Twitter.com, SMS, or Gtalk. But that's precisely what worries me about Twitter.
Why? There simply is no need for me to visit Twitter.com because it is so well integrated into my Blackberry and my Gtalk client. When reading, I enjoy Twitter in a passive matter. And when posting, I still do it remotely via Gtalk. But ultimately, I want a compelling reason to visit Twitter (and I'm willing to do so) - but I haven't yet found one. And if Twitter is going to monetize me as a user - it's important.
Important because the opportunity to monetize me as a user is greater - but more important because if ads were introduced into my Gtalk client or my Flash widget, I would consider it intrusive and likely leave.