It "could" be an interesting feature BUT, 2 things:
- please, do not select the first item of the list: most of the time, I don't want the first hit. And it's totally incompatible with Instant Preview.
- do not auto make the auto-completion.
In my opinion, the default behavior must be a default [search / go to URL] operation as Google Chrome does. I think this is the more natural use of the address bar.
If I want to keep the auto-completion suggestion, then I press right arrow.
If I want to select an item in the list, then I press top/bottom arrows.