I have the same issue too – I installed 2.7.1 and it did weird things – i.e. it dod not complete setup properly and created the tables but with no user in the users table. To get round that I copied the user info from wp_usermeta and wp_users in another blog I have and pasted that to the same tables in the new blog.
That let me log in, but it looks as though there are some issues with “permissions” as both the add new user link is missing and I cannot delete any posts, if I try I get a message saying that “You are not allowed to delete this post”.
Is this a missing setting or setting from wp_usermeta I wonder or something else?
Thanks,
C