Despite the number of avenues of communication on an Internet-equipped mobile phone, there are still one or two needs that remain uncatered for. Aside from MMS for instance, there's no iChat client to accompany you on long train trips.
It's arguable that the iPhone keyboard and interface generally don't lend themselves to the medium but the mobile version of meebo not only supports AIM, Yahoo! and its own service but also MSN, Google Talk, Jabber and ICQ. Enough surely for the biggest chatterboxes.
And it's possible to set a predefined or custom status. Rather than presenting multiple chats as tabs there's a small button at the bottom of each chat that returns to the contact list.
Any recent messages are listed below contact names, so you can keep a quick eye on conversations. It's a reasonable workaround given that tabs would be cumbersome in the available screen space. It's sufficient for participating in a limited number of chats at one time too.