By default, the Add Bookmark button (+) is attached to the URL address in Safari 4. There isn't an option to turn it off, but there's a workaround:
- Select the View » Customize Toolbar... menu option.
- Drag the standalone Add Bookmark button to the toolbar; it's in the top row. This will break the (+) off the address bar.
- Drag the Add Bookmark button off the toolbar. It will poof and leave you with a plain address bar.