I have almost a dozen macs with mini-display ports and use them in a wide variety of different scenarios. So long as you realize that OSX (like Vista/Win7) obeys EDID religiously and keep the appropriate spoofers around you shouldn't have a problem.
I believe that you are probably seeing EDID issues, although I must admit it has been a very long time since I have used G series hardware. Even so, assuming it is running OSX, it should obey the same rules.
That said, Current I have a variety of Macs with outputs encompassing DVI, Mini-DVI, and Mini-Display-Port. All of them need various adapters to get where they are going. We use mostly Analog Way and older TVOne boxes. They all need a spoofers to work properly.
You should make sure you keep both DVI EDID spoofers (Gefen makes the Detective) or VGA EDID spoofers (I keep Kramer VP-200K around) for these types of jobs/troubleshooting
Hope that helps,
Karl P