Have you also tried http://localhost/phpmyadmin
as both these address are Aliased to the actual phpMyAdmin folder.
If you think that this issue is related to the beta version of XAMPP for Windows then there is a feedback form on the beta web page.
Why the beta version?
Have you tried using the full stable version 1.7.0 which you will be able to upgrade when the 1.7.1 beta becomes stable? - there is no upgrade from a beta version to the stable version on completion of the beta testing program.
I see you edited your post whilst I was compiling this reply.
I am please it is working for you now.