For both performance and supportability reasons, it is extremely inadvisable to put Exchange mailboxes/private stores/public stores on anything other than a filesystem which is directly-connected to the NT server in question.
People do all sorts of odd things with NetApps just because they can - and don't get me wrong, I love my F740 - but this is just plain crazy. For your own peace of mind, don't do it.
So the netapp still chokes on lots of active small files i.e. like usenet news, etc? We were thinking about turning ours into a hardcore news spool..but I remember a few years ago there was some issue with the millions of < 64KB file sizes..
-M