One of the first things I noticed when using the latest GNOME GUI on Ubuntu Lucid Lynx was that there was no button to change the location bar from button based to text based.
In older version you had a “pencil” icon in Nautilus that allowed you to switch back freely but it’s now been removed. Unfortunately there is no standard method of reverting to the pencil icon.
There are two ways you can. To instantly toggle between button-mode and text-mode use CTRL+L.
If, like me, you never use button mode you can set the location to text mode permenantly via the gconf editor. The easiest way to do this is to ALT+F2, type in gconf-editor and hit Run.
In the configuration editor go to apps > nautilus > preferences then tick (check) always_use_location_entry. With that checked you’ll permenantly have the text based location bar.
You can also use / to open up a fresh (empty) location bar.