Apple - Add an up button to the Lion Finder?

Would you be willing to just use a keyboard shortcut instead?

⌘ Command↑ Up

Voilà.

Choose View > Show Path Bar to show the Path bar at the bottom of Finder windows.

Then, to go up one level, just double-click on the second-to-last item in the hierarchy. For example, I'd double-click on mdouma46 in the image below to go up one level.

Another option would be to save a script like this as an application in AppleScript Editor and drag it to the toolbar.

try
    tell app "Finder" to tell window 1 to set target to container of target
end try