"This rounded corner on ie7/winxp doesn't look exactly the same as the rounded corner on my iphone4!" isn't the type of complaint I hear from clients any more, but font renderings, aliasing, shape of rounded corners, etc are all things I've had to deal with in the past.
We're now entering mobile devices/tablets, which will bring with it its own version of the same stuff. It was easier to tell someone "that's netscape, this is opera - they're different, and will look a little different*" than it will be to say "this is android, and that's android, but they don't work the same".