Oh, and can you make it alpha-transparent when it's not the active window ?

I don't understand the question.

I think he was talking about having it support alpha transparency such that when it's the active window, it's opaque, but when it's not (not focused) it's transparent (at least partially, 100% transparency isn't all that useful.)

Trillian supports this, if you wanna take a look at how it works.