It might be helpful if you could describe the Visual VoiceMessage functionality on the iPhone, as I doubt many of us have even seen it.
On the other hand, when I read your posting, I was wondering if it's a program which will behave like an answering machine, resulting in a recording of the caller's voice within the device's local storage in some kind of media file format.
If this is true, I'd also love to have something similar on the 6700. From what I've read, while WM6 provides some interesting APIs, the 6700 doesn't have the necessary hardware links between the 'phone' & 'PPC'. If I'm wrong - GREAT - but tell me why & prove it