Set iPhone ringtone for filtered email messages
I don't believe there's a native way to do this, but you could set up server side rules (via Outlook Web Access, server side so they always process) to forward particular messages to a notification service, such as Boxcar (www.boxcar.io), then configure individual sounds using their iOS app.

Here's a page that shows how to setup forwarding rules in OWA (chosen randomly via Google):
http://programs.cocc.edu/OWA/FW/default.aspx

One catch with this is that, as far as I can tell, the Rules functionality is only available in Internet Explorer, at least with Exchange 2007. I tried this in Chrome and Safari on my Mac at home and the option isn't available.  I've heard non-IE browser support is better for Exchange 2010. If you don't have access to IE, Google a bit to see if you can find any tips for using IE 8 or 9 on OS X. I found one, but I'm not exactly sure of it's legitimacy, so I chose not to post a link.