There's iChat from years go. As it flounders, we gained Facetime last year. Facetime was Wi-Fi only but that might change depending whether a carrier allows Facetime chat over 3G. And now, we've got iMessage coming to iOS 5 only. And three apps don't work with each other.
This post from The iPhone Blog summarizes the situation well. For me, I think the best way to solve the situation is to allow iChat take over Facetime and iMessage duties on the Mac. On the iOS side, Apple should integrate Facetime into iMessage for simplicity sake and to eliminate an extra and unnecessary icon.