Any REAL good Mac/iPhone combo IM applications out there?
I'm not sure about the unified session requirement - I've never seen that in any IM client.

However, I find Adium (http://adium.im/) is the best Mac IM application (and it's open source!).

As for the iPhone, have you considered IM+ (http://www.shapeservices.com)?  It works quite well for me.