I just use custom ringtones to distinguish my in-callers, however, you might be on to something. I haven't heard of anything like that, but I'll keep looking. Interesting... Very interesting...
If you got the Jawbone Icon earpiece it anounces the number thats calling you. Surely it wouldnt be so hard to make a app to do the same, im pretty supprised no one has done this yet (that I know of).
Google "AT&T Labs Text To Speech." This app will create ringtones using human voices. You can then assign the ringtone to each number in your address book.
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voxtonepro.com has been doing just this for years. You can make custom professionally recorded ringtones that will annouce anything you type in, then you can set as your caller ID tones.