We wrote about Meebo's iPhone compatibility right after the iPhone was launched—and it wasn't so great. Consider that a thing of the past. Meebo's just released an iPhone-only IM webapp that's just as good as Meebo for the desktop. We've actually tested it on our iPhones, and can say it's the best chat app on the iPhone yet, partially thanks to the fact that it is tied in to the Meebo on the desktop.
You get the standard Meebo IM networks you get on the desktop. You can even log in to the same Meebo account and use the same settings, including away messages and previous messages. Actually chatting is great as well, allowing you to keep the keyboard up while chatting, which is super convenient. They wanted to make this as light as possible, which means some features are stripped out (like adding contacts). But all the important ones you need to hold a conversation are there. [Meebo]