How do I make a horizontal user login block for the top user region?
There are many many ways to do this, there is even a screencast and a module to do this. I would personally do this from the theme layer using css, or if I was using Fusion as a base theme, I would do this from the block configuration (thanks Skinr!)

I'm adding some links that will help you out.

http://drupal.org/project/userloginbar

http://ericlondon.com/changing-user-login-block-horizontal-layout

http://www.youtube.com/watch?v=XMyOX6v-k3Y

http://www.victheme.com/blog/drupal-7-creating-horizontal-login-bar-without-module