I don't know what the deal is with comments.
I haven't changed anything.
I've been able to comment when I was already signed in and by signing in inside the comment box, so I don't know what the problem is.
It looks like I'm not the only one. I'm guessing it's a beta issue. Others have brought it up in the help section, so hopefully it will be resolved soon.
***Blogger update*** "Logging in with an old Blogger account to post a comment on the new Blogger is giving a “please try again later” error. Until we fix this, it may work to log in first at http://www.blogger.com/login.g, and then go to the comments page on the new version of blogger in beta."
I don't know if this helps any of you...