Slowly all Google services are moving to Google accounts. Right now Blogger is switching over: Beginning today the new Beta version (why Beta? has blogger been alpha before?) is advertised in the Blogger dashboard and blogs can be converted with just a few clicks - and logging into an existing or opening a new Google account.
So now there is one more Google project connected to all the other Google services - sometime you wonder if it is good to put so much information about yourself (mail, calendar, documents) into the hands of one company.