There are some differences between the implementations of the search field in the Finder, iTunes, Mail, and Safari. Consistency being one of the hallmarks of Apple software, I think this ought to be fixed.
Based on the screenshot at Apple’s 10.3 Finder info page, it looks like it displays the scope of the search in the inactive input box. I believe the magnifying glass menu is used, as in Mail and iTunes, to change the scope of the search.
iTunes displays a magnifying glass on the left of the search field which, when clicked, allows the user to select what field they would like to search: all, artists, albums, composers, or songs. The type of query to be performed is listed immediately beneath the input box. Because of the iTunes interface layout, placing the query type label beneath the entry box rather than inside is acceptable because it does not use additional screen space. With that said, it is still not consistent with other applications.
Mail uses the menu to allow users to select the field and scope of their queries; the entire message, subject line, to or from in either the currently selected mailbox or all of their mailboxes. When the search field is not in focus, it displays the name of the field selected for searching (message, subject, etc.).
Safari takes a different approach, using this left side magnifying glass menu as a history menu, displaying previously run queries when selected. When not focused, it also displays a text label for the type of query to be performed (always “Google”).
While they are both functional, the method used by Safari assumes that Google’s general search will always be the search engine used. The implementations in Firebird and Camino are open to extension, allowing the developers to add engines without having to change the existing UI.
The problem is one of displacement: if Apple ever adds the ability to select from multiple search sites/types, what happens to the history feature? I believe auto-complete could be used in this case. It is used in website search input boxes, so why not in Safari’s Google Search?
Alternately, the UI could be substantially overhauled. The current UIs are designed so that the option to select the engine is placed first. Users probably do not think of what engine they want to search first – they have a query in mind, not an engine. Placing the search engine selection menu on the right of the input dialog makes more logical sense to me, better reflecting the thought process of using search services. Also, rather than search queries being submitted only after the user hits the Enter/Return key, why not automatically submit the query once an alternate engine is selected?
Editing Note: this entry was revised on October 7/8 to include information on the 10.3 Finder and Mail’s search field interfaces.