The concept of versions takes a step away from the filesystem and into abstract managed storage. If you have used a Newton OS-based device before, where there is no filesystem, you'll appreciate why "Save As..." makes no sense. iOS uses the same concept to manage documents in an application—you open documents from a list of available ones, and then edit them. No closing (you close a document when you quit the app or open another document), and no saving. They are just there as you last left them.
Versions in OS X Lion takes the concept of a document as an atomic "object" rather than something that is in memory and has an associated "file" on disk. The idea is that Mac applications work more like iOS, and Newton OS before it, and OpenDoc before that, and the Apple Lisa before that, in that documents are objects that you open and edit. No closing, no saving.
But when I say "no saving", I should say that saving is done without your telling so—they are automatic. To allow the system to track the list of automatically saved changes so that you can "undo" or "revert" to previous states, it keeps versions of your documents.
The change in concept means that documents must be named on creation—the system won't let you create an anonymous, memory-only document like you could on the Mac since 1984. You can change the name of the document once you have created it, and the default name could be "untitiled", but the name must exist, and it is the app that has the responsibility of creating the document's name.
It also means that the concept of "saving as a new name" in Lion is fundamentally different. In the early days of computing, documents in memory and documents saved on disk were two different entities, ie., documents on disk were copies of documents in memory at a point in time. There is no one-to-one association between a document in memory and a document on disk.
Documents in Lion and iOS go back to the days of Lisa, OpenDoc and Newton OS and take on the semantic that a document in memory now has an association with a document in storage, where storage can include a disk. This is an important distinction, because it may be that the document in memory and the document in storage is actually one and the same object, particularly where documents exist on computers with flash memory for storage.
In order to make something like a "Save As..." work, you need to make a copy of the document. This is where Lion's "Duplicate" command comes in (you'll see that command in the File menu of Keynote, Numbers and Pages for Lion). To to save a document under a new name, you have to actually duplicate (make a copy of) it first, and give the copy a new name. Duplicating a document makes a copy of the actual object, wherever it may exist (in memory, on disk, in storage... whatever), because that object has an existing association that needs to be replaced with a new one when you make the duplicate, ie., a document and its copy are two separately identifiable atomic objects.
This is distinct to versions of a document, which are actually implemented as different copies of a document in time that appear under the same name, and may also be identified as the one atomic object in storage. This allows Lion to be able to "... share only the most recent copy, not all the hard work it took to get there" when you share a document by copying it over to another Mac or to another user.
So to cut a long story short:
• documents are now atomic objects that carry associations regardless of where they exist on your system: on disk, on the network, in memory or on flash storage,
• to save a document under a new name, you need to make a copy of the document first and then change the new copy's name—this is handled by the "Duplicate" command in the "File" menu of Lion applications.
• versions of a document are not duplicates of it. All versions of a document are essentially the one and the same document, except that they track the changes to the document as they appear in different points in time.
• Lion's document management system is intended to support the concept of auto-saving where a document can be changed both in memory and in storage regardless of where in the system it lives.
As a sidenote:
• the Lisa also did have a "Duplicate..." command in order to save documents as copies under new names <http://toastytech.com/guis/lisaos3desk.gif>. Also note that Lisa doesn't have a "Save" command, but it does have a "Save and Put Away" command, which saves the document away and closes it (saving whilst a document is open is an automatic process).
• the Newton OS requires that users use the routing button (which looks like an envelope) to duplicate documents, whether in an application or on the Newton OS Extras drawer.
• OpenDoc made things more complicated by providing a Save command, since saving was not an automatic process, but it also provided a "Duplicate..." command for creating copies of a document under a new name.
• Keynote, Numbers and Pages in iOS shows you no filesystem, and lists documents that you can open, delete or duplicate (using icons on the top bar). There is no "Save As...".
I hope this makes things clearer as to what is intended as the document model for Lion, and that people can appreciate the reasoning behind the differences.
—tonza