Presume the iPhone web app would work so you could try this (in an Apple store perhaps if you don't have a Touch yet)
http://www.mundu.com/im/iphone/
JiveTalk is an alternative:
http://www.beejive.com/iphone/
Can't vouch for either as I don't have an iPhone or a Touch - just a couple of links I've collected while meandering around the net waiting for the iPhone to be released in the UK. I think I read somewhere that meebo is iPhone friendly now too.