Microsoft, Sun, and others are already hard at work on an identity framework. And then there's OpenID, which doesn't rely on a plugin, just some fairly simple server-side stuff that allows some other server that knows you to vouch for you. The plug-in, then, would just be your own blog server's OpenID server. No need for anything on your browser, which is a darned good thing because some people blog from cell phones and other devices that don't support user-installed software.

There's a bit of overhead, but it's probably no more than you'd have with the current "friend me and I'll add you back" stuff. Probably less, because you could make it into a browser plug-in that would work everywhere, not just on one blogging site.