Mundu IM: iPhone Edition, Chat Till Your Thumbs Bleed
Mundu has a long pedigree in the world of business-class mobile instant messaging software. I have used their Mobile Messenger client on PalmOS and Windows Mobile - it works like magic. So it didn't take long for their developers to show some iPhone love.
The company has announced a web based service that fully supports iPhone, via Safari of course (we've heard this line before). To access the web client simply point your iPhone to iphone.mundu.com. Login to any existing IM accounts (Yahoo, Gtalk, AIM, or Windows Live Messenger protocols supported), and text your way to repetitive stress injury.
The service is in beta, so don't look surprised when Safari chokes and drops you back to the homescreen.