I heard about comment sharing or pooling across blogs from Leo Laport a month or two ago but now we are getting closer with our new product trying out http://www.intensedebate.com/ and http://www.cocomment.com/ seemed a good idea. Also using Gravitar sites well with me.
I am trying intensedebate on this blog even if it will not get all the comments it could. They have a WordPress version coming so will have to wait for that. I like the idea of not having to register lots of places to comment and also to comment on idfferent pages on different sites about the same thing.