Here's how to essentially make Google your OpenID provider if you use Blogger:
* Go to http://draft.blogger.com (this doesn't work through the regualr Dashboard)
* Click "Edit Profile" in the upper right
* Check the box that says "Enable OpenID for Blogs"
* Scroll down and click "Save Profile"
That's it. I tested it using my blog.rosshollman.com to ensure that it worked for custom domains, and it works just fine. Also, you can use any blog associated with your Blogger account as your OpenID.
Presumably Google will role this out at some point to anyone with a Google account, but if you want it now, this is how you get it.