1) All browsers with Flash players 2) Any "web connected" set top device/TV 3) About 50% of HTML5 video compatible browsers
With this move by Google #3 falls to 25% or less. And they're looking to add WebM support to #2. At some point consumer demands you support both because their shiny new Macbook Air/Chrome OS tablet/TV doesn't play video. Good move by Google to justify their $125M investment in On2, but end users will end up being inconvenienced as the splintering continues.
This is just going to feel like a regression to most end users who really don't give a crap about who holds what patents.