It's down to individual developers. I'd suspect you'll see most popular titles get them and those developers who focus on pleasing consumer will also no doubt step up - so already Uncharted and Burnout have been announced for example.
I doubt all old games will get them as why would a developer put them into an old game that didn't sell too well?
The focus will be on new games for Trophies I suspect.
Try to be reasonable... its easier than you think...