BlogThis! is not yet supported in beta (it won't accept beta login information). This applies both to the original BlogThis! link as well as the Blogger button on the Google Toolbar.
Update (9/12): This is now being worked on.
Update (1/4): This is now fixed. The BlogThis! bookmarklet and Google Toolbar button link to a login box that accepts either old Blogger or new Blogger (i.e. Google Accounts) logins.