To quote the jQuery folks:
Our goal is for 1.9 and 2.0 to be interchangeable as far as the API set they support. When 2.0 comes out, your decision on which version to choose should be as simple as this: If you need IE 6/7/8 support, choose 1.9; otherwise you can use either 1.9 or 2.0.
I’m happy that someone is taking a step to eliminating support for these archaic browsers.
IE is always annoying to support, as it doesn’t handle things correctly, and requires “babysitting” whenever you want to do anything special on it (ie HTMLshiv).
According to the post, this won’t happen until early 2013.
The nice thing, for those of us who will have to support IE, is that jQuery 2.0 will support all APIs that jQuery 1.9 will. It sounds like it will just be cleaner and faster. You could (at least for the foreseeable future) implement websites in jQuery 2.0 and fall back to jQuery 1.9 when the user is on IE.
Check out the full post, jQuery Core: Version 1.9 and beyond.