I've thought about that too. Wouldn't it be cool if his voice suddenly switched to the GOOG411 voice? Or any of the modern speech synthesis voices? If I was him, I would definitely switch. I bet there are plenty of companies that would donate the system to him. But like simon666 said, since everyone knows that voice as him, I can see why he wouldn't want to change.
I've also thought about it too. Did anyone see the show where the MIT guy is constantly upgrading his prosthetic leg? I could easily see someone like Hawking taking the opportunity to play with new synthesis tools. Maybe he's afraid of falling in the uncanny valley.