Also, many of these games were designed as PC games from the ground up and are thus optimized for mouse and keyboard input. I couldn't imagine trying to play an RTS on a console. Or trying to read all the text on a game like Planescape on a TV screen.
I do think the touch screen and gyro in the iPad could make for some good control on some of the games, but you would need access to the source code to implement that. Something gog doesn't get from the dev/publisher, and in some cases is completely lost.
On top of that, I don't see there being a lot of demand to play old PC games on modern consoles or iPads. Most people who use those devices want to play newer games that were designed for their system of choice. Most people who are interested in playing old PC games want to play them on a PC, their native platform. And before you argue that the iPad would make them portable, most of the games here can be run on a netbook, which is just as portable as an iPad and often much cheaper.