We moved it because
It's the least used item on the header as measured by HTTP requests.
We need more room on the header, as we've added SE global nav, chat, meta, and two kinds of flags there. For people with "long" names like Rebecca Chernoff it was causing the header to wrap.
The link was moved to your user page, or you can always type in
/users/logout in the address bar.
Edit - the link is not on the user page any more, it's now located on the mouse-over pop-down menu available when you hover the mouse over your name in the header.