This depends on the site functionality.
Administration site is typically something you always have to log in (no access unless you log in), so this would typically be center screen.
Other sites that have some valid functionality for anonmous users (like Google and Yahoo), it would be more natural to have a login section at the top right. This is the case where you can browse around without necesarrily have to log in. If you need some user specific functionality, you need to log in to get access to more.
A third option is if the site generally does not need login for most functionality, and login is just required for some advanced functionality, which is only availabe to a selected group of users (example: editors of the site content). In this case it would be natural to just have a login link/button some place, and when click on that, you get a login screen in center.