I’ve installed if for all zero-comment posts (which includes all new posts), so old posts with comments still have the old WordPress commenting system. This may involve you having to create a Disqus account, but the idea is that you can use that account across blogs.
I am also setting up Disqus on
so you only have to create your account once for both places. Leo Laporte has also set up his blog to use it.
Maybe it will spread everywhere. The idea is that your comments belong to you, and Disqus makes it easier to follow people, as opposed to blogs.