That's very cool folks,
finally a full IM has came out for iPhone. You need to install Fring on your iPhone by adding a new source (http://fring.com/iphone.xml), make a simple free registration and here we go, that's all.
After that you may configure your own IM like for example Skype, MSN, SIP, Google Talk, Twitter, Yahoo and AIM. The software rests on background mode also when you put on sleep the display and when you push on Home button, in that way you don't need to let fired on your iPhone .
Of course it sucks a lot of battery power because it lets the wireless communication switched on and the process alive but to me, it's a great software; I definitely suggest it !
Actually I've just some doubts about the security of the system, I had no chances yet to verify the communication, the encryption level (if there is) and the application accuracy, please if someone of you, is going to test these stuff let me know as soon as possible, we might do that together, I am really scared to see IM passwords moving arount the word without any kind of encryption.