That's what Open Id is for, and other tools would exist too. But there probably isn't much incentive for anyone to start really building that, because the core of every site like this (especially LJ and such, general blog sites not so much) is to build their own community, with their own users and stuff. Also, many communities have quite different "cultures" to them, and mixing them together would be problematic (not technically, but socially). So I don't think it's going to happen.