We are in the process of ditching Cisco Unity for Exchange 2007 Unified Messaging; however, I just came to realization that Exchange doesn't seem to provide a way to upload a WAV file to be used as a voicemail greeting. This could be a problem, since we have several mailboxes that use professionally recorded greetings.
I found this article which shows how to access the data through MAPI, but it does not provide a way to upload new files.
Note, this is not for the auto attendants, but for actual voicemail greetings.
There's no straightforward/easy way to do this. The user's greeting is stored as an associated content item, which means you'll have to write code to work with those items. You can do it using Extended MAPI, Redemption, or potentially EWS (although I'm not 100% sure that EWS works with associated items).
MS intended greetings to be managed by individuals, which is why there's no way to do this directly through the admin tools. It wouldn't be super hard to write a small tool to do this, though.